Bump sphinx-rtd-theme from 1.2.0 to 1.3.0

Bumps [sphinx-rtd-theme](https://github.com/readthedocs/sphinx_rtd_theme) from 1.2.0 to 1.3.0.
- [Changelog](https://github.com/readthedocs/sphinx_rtd_theme/blob/master/docs/changelog.rst)
- [Commits](readthedocs/sphinx_rtd_theme@1.2.0...1.3.0)

---
updated-dependencies:
- dependency-name: sphinx-rtd-theme
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

docs/requirements.txt

@@ -1,6 +1,6 @@
 urllib3==1.26.15 # https://github.com/readthedocs/readthedocs.org/issues/10290
 sphinx==1.8.6
-sphinx-rtd-theme==1.2.0
+sphinx-rtd-theme==1.3.0
 sphinxcontrib-napoleon==0.7
 invoke==2.0.0
 jinja2==2.11.3