Fix Sphinx version requirements

[trixmoe]

The existing requirements file causes Sphinx to error out when trying to build.

$ make dirhtml
sphinx-build -b dirhtml -d _build/doctrees   . _build/dirhtml
Running Sphinx v3.5.4
/dir/wpull/.venv/lib/python3.5/site-packages/requests/__init__.py:89: RequestsDependencyWarning: urllib3 (1.26.9) or chardet (2.3.0) doesn't match a supported version!
  RequestsDependencyWarning)

Extension error:
Could not import extension sphinxarg.ext (exception: cannot import name 'Directive')
make: *** [dirhtml] Error 2

The Sphinx version was not specified properly, which causes it to update past a supported version, considering the other dependencies.

I'm testing this using a 3.5 venv, and installing requirements-sphinx, then running make html and make dirhtml.

There are 14 warnings, but now it does not fail outright.

This works towards fixing #495, but does not address the issue itself.

[trixmoe]

I added a bunch of commits I shouldn't have which is now fixed.