docs: Require sphinx-rtd-theme compatible with Sphinx 7

* Update lower bound on sphinx-rtd-theme to v1.3.0rc1 to ensure
  compatability with Sphinx v7+.
   - c.f. https://github.com/readthedocs/sphinx_rtd_theme/issues/ 1463
   - As soon as sphinx-rtd-theme v1.3.0 is released on PyPI the lower
     bound should be updated to reflect this stable release. The release
     candidate is being used to unbreak CI.
* Update lower bound of Sphinx to v7.0.0 for consistency.

pyproject.toml

@@ -109,7 +109,7 @@ docs = [
     "sphinx>=5.1.1",  # c.f. https://github.com/scikit-hep/pyhf/pull/1926
     "sphinxcontrib-bibtex~=2.1",
     "sphinx-click",
-    "sphinx_rtd_theme",
+    "sphinx-rtd-theme>=1.3.0rc1",  # c.f. https://github.com/scikit-hep/pyhf/pull/2271
     "nbsphinx!=0.8.8",  # c.f. https://github.com/spatialaudio/nbsphinx/issues/620
     "ipywidgets",
     "sphinx-issues",