Bump sphinx from 3.3.0 to 3.3.1 in /docs (cjolowicz#670)

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 3.3.0 to 3.3.1.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/3.x/CHANGES)
- [Commits](sphinx-doc/sphinx@v3.3.0...v3.3.1)

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{{cookiecutter.project_name}}/docs/requirements.txt

@@ -1,3 +1,3 @@
-sphinx==3.3.0
+sphinx==3.3.1
 sphinx-click==2.5.0
 sphinx-rtd-theme==0.5.0