Sphinx 2.1.0 has a bug [0] that causes whitespace to be excluded in
outputted docs. This change updates Sphinx to peg any version >2.1.0.
Safety dependency vulnerability checks now will also cover the doc
requirements.txt.
[0] https://github.com/sphinx-doc/sphinx/issues/6440
Change-Id: I35f1acf4385821969ffa7c9807cf209a59c1d73e