Update sphinx-rtd-theme requirement from ~=1.0.0 to ~=2.0.0

Updates the requirements on [sphinx-rtd-theme](https://github.com/readthedocs/sphinx_rtd_theme) to permit the latest version.
- [Changelog](https://github.com/readthedocs/sphinx_rtd_theme/blob/master/docs/changelog.rst)
- [Commits](readthedocs/sphinx_rtd_theme@1.0.0...2.0.0)

---
updated-dependencies:
- dependency-name: sphinx-rtd-theme
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

requirements.txt

@@ -5,6 +5,6 @@ Sphinx~=5.0
 doc8~=0.10.1
 docutils~=0.17.1
 reuse~=0.14.0
-sphinx-rtd-theme~=1.0.0
+sphinx-rtd-theme~=2.0.0
 sphinxcontrib-spelling~=8.0.0
 sphinx-multiversion~=0.2.4