update docs requirements
sphinx == 3.2.1
git+https://github.com/python/python-docs-theme.git#egg=python-docs-theme
git+https://github.com/pypa/pypa-docs-theme.git#egg=pypa-docs-theme
-https://github.com/djungelorm/sphinx-tabs/releases/download/v1.1.13/sphinx-tabs-1.1.13.tar.gz
+sphinx-tabs == 1.1.13
# `docs.pipext` uses pip's internals to generate documentation. So, we install
# the current directory to make it work.
