Update sphinx requirement from ~=3.5.4 to ~=4.0.2 (#72)

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] <[email protected]>

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

Author: dependabot[bot]

requirements-dev.txt

@@ -12,6 +12,6 @@ flake8 ~=3.9.2
 pytest ~=6.2.4
 ipython ~=7.24.0
 numpydoc ~=1.1.0
-sphinx ~=3.5.4
+sphinx ~=4.0.2
 sphinx-copybutton ~=0.3.1
 sphinx_rtd_theme ~=0.5.2