Update sphinx requirement from ~=3.3.1 to ~=3.4.1 (#50)

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@v3.3.1...v3.4.1)

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

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

Author: dependabot[bot]

requirements-dev.txt

@@ -12,6 +12,6 @@ flake8 ~=3.8.4
 pytest ~=6.2.1
 ipython ~=7.19.0
 numpydoc ~=1.1.0
-sphinx ~=3.3.1
+sphinx ~=3.4.1
 sphinx-copybutton ~=0.3.1
 sphinx_rtd_theme ~=0.5.0