Update sphinx requirement from ~=4.1.2 to ~=4.2.0 (#84)

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@v4.1.2...v4.2.0)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-type: direct:development
...

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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 ~=6.2.5
 ipython ~=7.28.0
 numpydoc ~=1.1.0
-sphinx ~=4.1.2
+sphinx ~=4.2.0
 sphinx-copybutton ~=0.4.0
 sphinx_rtd_theme ~=1.0.0