Merge pull request #1059 from tskit-dev/dependabot/pip/python/requirements/CI-docs/sphinx-3.3.1

Bump sphinx from 3.2.1 to 3.3.1 in /python/requirements/CI-docs

Author: mergify[bot]

python/requirements/CI-docs/requirements.txt

@@ -5,7 +5,7 @@ h5py==3.1.0
 jsonschema==3.2.0
 numpy==1.19.4
 PyGithub==1.53
-sphinx==3.2.1
+sphinx==3.3.1
 sphinx-argparse==0.2.5
 sphinx-issues==1.2.0
 sphinx_rtd_theme==0.5.0