Update sphinx-rtd-theme requirement from ~=1.0 to ~=1.3

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...1.3.0)

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

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

docs/requirements.txt

@@ -1,3 +1,3 @@
 sphinx~=5.3
-sphinx-rtd-theme~=1.0
+sphinx-rtd-theme~=1.3
 sphinxcontrib-napoleon~=0.7