Update sphinx requirement from ~=3.5.4 to ~=4.0.2 (#724)

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/4.x/CHANGES)
- [Commits](sphinx-doc/sphinx@v3.5.4...v4.0.2)

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements-dev.txt

@@ -7,5 +7,5 @@ ipython ~=7.24.0
 matplotlib ~=3.4.2
 numpydoc ~=1.1.0
 requests-mock ~=1.9.3
-sphinx ~=3.5.4
+sphinx ~=4.0.2
 sphinx_rtd_theme ~=0.5.2