Update sphinx to ~=3.1.1, sphinx-rtd-theme to ~=0.5.0 (#601)

Updates the requirements on [sphinx](https://github.com/sphinx-doc/sphinx) to permit the latest version.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/3.x/CHANGES)
- [Commits](sphinx-doc/sphinx@v2.4.4...v3.1.1)

Updates the requirements on [sphinx-rtd-theme](https://github.com/rtfd/sphinx_rtd_theme) to permit the latest version.
- [Release notes](https://github.com/rtfd/sphinx_rtd_theme/releases)
- [Changelog](https://github.com/readthedocs/sphinx_rtd_theme/blob/master/docs/changelog.rst)
- [Commits](readthedocs/sphinx_rtd_theme@0.4.3...0.5.0)

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>
Co-authored-by: Rob Brackett <rob@robbrackett.com>

Author: dependabot-preview[bot]

requirements-dev.txt

@@ -8,5 +8,5 @@ ipython ~=7.16.1
 matplotlib ~=3.2.2
 numpydoc ~=1.0.0
 requests-mock ~=1.8.0
-sphinx ~=2.4.4
-sphinx_rtd_theme ~=0.4.3
+sphinx ~=3.1.1
+sphinx_rtd_theme ~=0.5.0