Update sphinx requirement from ~=3.1.2 to ~=3.2.1 (#628)

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@v3.1.2...v3.2.1)

Signed-off-by: dependabot-preview[bot] <[email protected]>

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

Author: dependabot-preview[bot]

requirements-dev.txt

@@ -8,5 +8,5 @@ ipython ~=7.18.1
 matplotlib ~=3.3.1
 numpydoc ~=1.1.0
 requests-mock ~=1.8.0
-sphinx ~=3.1.2
+sphinx ~=3.2.1
 sphinx_rtd_theme ~=0.5.0