Update sphinx requirement from <8,>=6 to >=6,<9 (#3269)

Updates the requirements on
[sphinx](https://github.com/sphinx-doc/sphinx) to permit the latest
version.
<details>
<summary>Release notes</summary>
<p><em>Sourced from <a
href="https://github.com/sphinx-doc/sphinx/releases">sphinx's
releases</a>.</em></p>
<blockquote>
<h2>Sphinx 8.0.0</h2>
<p>Changelog: <a
href="https://www.sphinx-doc.org/en/master/changes.html">https://www.sphinx-doc.org/en/master/changes.html</a></p>
<h2>Dependencies</h2>
<ul>
<li><a
href="https://redirect.github.com/sphinx-doc/sphinx/issues/12633">#12633</a>:
Drop Python 3.9 support.</li>
</ul>
<h2>Incompatible changes</h2>
<ul>
<li>
<p>Remove deprecated functions from <code>sphinx.util</code>:</p>
<ul>
<li>Removed <code>sphinx.util.path_stabilize</code>
(use <code>sphinx.util.osutil.path_stabilize</code>).</li>
<li>Removed <code>sphinx.util.display_chunk</code>
(use <code>sphinx.util.display.display_chunk</code>).</li>
<li>Removed <code>sphinx.util.status_iterator</code>
(use <code>sphinx.util.display.status_iterator</code>).</li>
<li>Removed <code>sphinx.util.SkipProgressMessage</code>
(use <code>sphinx.util.display.SkipProgressMessage</code>).</li>
<li>Removed <code>sphinx.util.progress_message</code>
(use <code>sphinx.util.display.progress_message</code>).</li>
<li>Removed <code>sphinx.util.epoch_to_rfc1123</code>
(use <code>sphinx.http_date.epoch_to_rfc1123</code>).</li>
<li>Removed <code>sphinx.util.rfc1123_to_epoch</code>
(use <code>sphinx.http_date.rfc1123_to_epoch</code>).</li>
<li>Removed <code>sphinx.util.save_traceback</code>
(use <code>sphinx.exceptions.save_traceback</code>).</li>
<li>Removed <code>sphinx.util.format_exception_cut_frames</code>
(use <code>sphinx.exceptions.format_exception_cut_frames</code>).</li>
<li>Removed <code>sphinx.util.xmlname_checker</code>
(use <code>sphinx.builders.epub3._XML_NAME_PATTERN</code>).</li>
</ul>
<p>Patch by Adam Turner.</p>
</li>
<li>
<p>Removed <code>sphinx.util.osutil.cd</code>
(use <code>contextlib.chdir</code>).
Patch by Adam Turner.</p>
</li>
<li>
<p>Removed <code>sphinx.util.typing.stringify</code>
(use <code>sphinx.util.typing.stringify_annotation</code>).
Patch by Adam Turner.</p>
</li>
<li>
<p><a
href="https://redirect.github.com/sphinx-doc/sphinx/issues/12593">#12593</a>:
Raise an error for invalid <code>html_sidebars</code> values.
Patch by Adam Turner.</p>
</li>
<li>
<p><a
href="https://redirect.github.com/sphinx-doc/sphinx/issues/12593">#12593</a>:
Raise an error in <code>Theme.get_config</code> for invalid sections.
Patch by Adam Turner.</p>
</li>
<li>
<p><a
href="https://redirect.github.com/sphinx-doc/sphinx/issues/11693">#11693</a>:
Remove support for old-style <code>Makefile</code> and
<code>make.bat</code>
output in <code>sphinx-quickstart</code>.</p>
</li>
<li>
<p><a
href="https://redirect.github.com/sphinx-doc/sphinx/issues/11693">#11693</a>:
Remove the <code>--no-use-make-mode</code>, <code>-M</code>,
<code>--use-make-mode</code>, and <code>-m</code> options
from <code>sphinx-quickstart</code>.</p>
</li>
</ul>
<!-- raw HTML omitted -->
</blockquote>
<p>... (truncated)</p>
</details>
<details>
<summary>Changelog</summary>
<p><em>Sourced from <a
href="https://github.com/sphinx-doc/sphinx/blob/master/CHANGES.rst">sphinx's
changelog</a>.</em></p>
<blockquote>
<h1>Release 8.0.0 (released Jul 29, 2024)</h1>
<h2>Dependencies</h2>
<ul>
<li><a
href="https://redirect.github.com/sphinx-doc/sphinx/issues/12633">#12633</a>:
Drop Python 3.9 support.</li>
</ul>
<h2>Incompatible changes</h2>
<p>.. rst-class:: compact</p>
<ul>
<li>
<p>Remove deprecated functions from <code>sphinx.util</code>:</p>
<ul>
<li>Removed <code>sphinx.util.path_stabilize</code>
(use <code>sphinx.util.osutil.path_stabilize</code>).</li>
<li>Removed <code>sphinx.util.display_chunk</code>
(use <code>sphinx.util.display.display_chunk</code>).</li>
<li>Removed <code>sphinx.util.status_iterator</code>
(use <code>sphinx.util.display.status_iterator</code>).</li>
<li>Removed <code>sphinx.util.SkipProgressMessage</code>
(use <code>sphinx.util.display.SkipProgressMessage</code>).</li>
<li>Removed <code>sphinx.util.progress_message</code>
(use <code>sphinx.util.display.progress_message</code>).</li>
<li>Removed <code>sphinx.util.epoch_to_rfc1123</code>
(use <code>sphinx.http_date.epoch_to_rfc1123</code>).</li>
<li>Removed <code>sphinx.util.rfc1123_to_epoch</code>
(use <code>sphinx.http_date.rfc1123_to_epoch</code>).</li>
<li>Removed <code>sphinx.util.save_traceback</code>
(use <code>sphinx.exceptions.save_traceback</code>).</li>
<li>Removed <code>sphinx.util.format_exception_cut_frames</code>
(use <code>sphinx.exceptions.format_exception_cut_frames</code>).</li>
<li>Removed <code>sphinx.util.xmlname_checker</code>
(use <code>sphinx.builders.epub3._XML_NAME_PATTERN</code>).</li>
</ul>
<p>Patch by Adam Turner.</p>
</li>
<li>
<p>Removed :py:func:<code>!sphinx.util.osutil.cd</code>
(use :py:func:<code>contextlib.chdir</code>).
Patch by Adam Turner.</p>
</li>
<li>
<p>Removed :py:func:<code>!sphinx.util.typing.stringify</code>
(use :py:func:<code>!sphinx.util.typing.stringify_annotation</code>).
Patch by Adam Turner.</p>
</li>
<li>
<p><a
href="https://redirect.github.com/sphinx-doc/sphinx/issues/12593">#12593</a>:
Raise an error for invalid :confval:<code>html_sidebars</code> values.
Patch by Adam Turner.</p>
</li>
<li>
<p><a
href="https://redirect.github.com/sphinx-doc/sphinx/issues/12593">#12593</a>:
Raise an error in :py:func:<code>!Theme.get_config</code> for invalid
sections.
Patch by Adam Turner.</p>
</li>
<li>
<p><a
href="https://redirect.github.com/sphinx-doc/sphinx/issues/11693">#11693</a>:
Remove support for old-style :file:<code>Makefile</code> and
:file:<code>make.bat</code>
output in :program:<code>sphinx-quickstart</code>.</p>
</li>
<li>
<p><a
href="https://redirect.github.com/sphinx-doc/sphinx/issues/11693">#11693</a>:
Remove the :option:<code>!--no-use-make-mode</code>,
:option:<code>!-M</code>,</p>
</li>
</ul>
<!-- raw HTML omitted -->
</blockquote>
<p>... (truncated)</p>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/sphinx-doc/sphinx/commit/0e8a6383be58ed1528c26a924abb73c0b4672e69"><code>0e8a638</code></a>
Bump to 8.0.0 final</li>
<li><a
href="https://github.com/sphinx-doc/sphinx/commit/5750d0e3fe8905fb582a6db95512f28b4c26159e"><code>5750d0e</code></a>
[bot]: Update message catalogues (<a
href="https://redirect.github.com/sphinx-doc/sphinx/issues/12641">#12641</a>)</li>
<li><a
href="https://github.com/sphinx-doc/sphinx/commit/c1ed1e3b70609cceaea8557453a12224bea64b77"><code>c1ed1e3</code></a>
Use <code>Path</code> objects within <code>copyfile</code> (<a
href="https://redirect.github.com/sphinx-doc/sphinx/issues/12708">#12708</a>)</li>
<li><a
href="https://github.com/sphinx-doc/sphinx/commit/a4102a7507166460318a86a2d127d3cc9d257982"><code>a4102a7</code></a>
Specify encoding to avoid EncodingWarning</li>
<li><a
href="https://github.com/sphinx-doc/sphinx/commit/b0485f932309e910c8c9961dcd7177a7c661085a"><code>b0485f9</code></a>
Add a migration note for <code>isNewType</code></li>
<li><a
href="https://github.com/sphinx-doc/sphinx/commit/feeee57dbecf8f4425ae9a3467edc8c37c417e61"><code>feeee57</code></a>
[HTML theme] sphinx13: include the project name in home link (<a
href="https://redirect.github.com/sphinx-doc/sphinx/issues/12700">#12700</a>)</li>
<li><a
href="https://github.com/sphinx-doc/sphinx/commit/0cb3c0750666827a0894fdd067c44e4ecbac3dac"><code>0cb3c07</code></a>
Add a missing space</li>
<li><a
href="https://github.com/sphinx-doc/sphinx/commit/5f110f0fa05ce7db3de9773e7c2423a73bf12633"><code>5f110f0</code></a>
Fix test failures following Alabaster's change in default sidebars</li>
<li><a
href="https://github.com/sphinx-doc/sphinx/commit/08adbb3c18e4f010c3c6b81ce6013447c2b8f433"><code>08adbb3</code></a>
Fix positioning of logos on the homepage (<a
href="https://redirect.github.com/sphinx-doc/sphinx/issues/12684">#12684</a>)</li>
<li><a
href="https://github.com/sphinx-doc/sphinx/commit/e2843fda5289fc18bb3fd04f2396d472627a6b20"><code>e2843fd</code></a>
Bump Ruff to 0.5.5</li>
<li>Additional commits viewable in <a
href="https://github.com/sphinx-doc/sphinx/compare/v6.0.0...v8.0.0">compare
view</a></li>
</ul>
</details>
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pyproject.toml

@@ -116,8 +116,8 @@ demo = [
   "ipython",
 ]
 doc = [
-  "sphinx>=6,<8",
-  "myst-parser==4.0.0",
+  "sphinx>=6,<9",
+  "myst-parser",
   "Jinja2",
   "alabaster",
   "sphinxcontrib-napoleon",