Update dependency Markdown to v3.6 (#1286)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [Markdown](https://togithub.com/Python-Markdown/markdown)
([changelog](https://python-markdown.github.io/changelog/)) | minor |
`==3.5.2` -> `==3.6` |

---

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---

### Release Notes

<details>
<summary>Python-Markdown/markdown (Markdown)</summary>

###
[`v3.6`](https://togithub.com/Python-Markdown/markdown/releases/tag/3.6)

[Compare
Source](https://togithub.com/Python-Markdown/markdown/compare/3.5.2...3.6)

##### Changed

##### Refactor TOC Sanitation

-   All postprocessors are now run on heading content.
- Footnote references are now stripped from heading content. Fixes
[#&#8203;660](https://togithub.com/Python-Markdown/markdown/issues/660).
- A more robust `striptags` is provided to convert headings to plain
text.
Unlike, the `markupsafe` implementation, HTML entities are not
unescaped.
- The plain text `name`, rich `html`, and unescaped raw `data-toc-label`
are
saved to `toc_tokens`, allowing users to access the full rich text
content of
    the headings directly from `toc_tokens`.
- The value of `data-toc-label` is sanitized separate from heading
content
before being written to `name`. This fixes a bug which allowed markup
through
in certain circumstances. To access the raw unsanitized data, retrieve
the
    value from `token['data-toc-label']` directly.
- An `html.unescape` call is made just prior to calling `slugify` so
that
`slugify` only operates on Unicode characters. Note that `html.unescape`
is
    not run on `name`, `html`, or `data-toc-label`.
- The functions `get_name` and `stashedHTML2text` defined in the `toc`
extension
are both **deprecated**. Instead, third party extensions should use some
combination of the new functions `run_postprocessors`,
`render_inner_html` and
    `striptags`.

##### Fixed

- Include `scripts/*.py` in the generated source tarballs
([#&#8203;1430](https://togithub.com/Python-Markdown/markdown/issues/1430)).
- Ensure lines after heading in loose list are properly detabbed
([#&#8203;1443](https://togithub.com/Python-Markdown/markdown/issues/1443)).
- Give smarty tree processor higher priority than toc
([#&#8203;1440](https://togithub.com/Python-Markdown/markdown/issues/1440)).
-   Permit carets (`^`) and square brackets (`]`) but explicitly exclude
backslashes (`\`) from abbreviations
([#&#8203;1444](https://togithub.com/Python-Markdown/markdown/issues/1444)).
- In attribute lists (`attr_list`, `fenced_code`), quoted attribute
values are
now allowed to contain curly braces (`}`)
([#&#8203;1414](https://togithub.com/Python-Markdown/markdown/issues/1414)).

</details>

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.github/workflows/mkdocs-requirements.txt

@@ -3,7 +3,7 @@ future==1.0.0
 Jinja2==3.1.3
 livereload==2.6.3
 lunr==0.7.0.post1
-Markdown==3.5.2
+Markdown==3.6
 MarkupSafe==2.1.5
 mkdocs==1.5.3
 mkdocs-macros-plugin==1.0.5