Fix source install Sphinx dependency issue

[PeterJCLaw]

See #52 (comment). There's an issue we don't understand where installing from source into a clean environment fails due to a sphinx import error. This seems to stem from some recent (ish) changes in setuptools/sphinx/python as it only impacts Python > 3.7 and recent setuptools/sphinx.

This can be worked around by installing sphinx first, which we do in CI (#55), but obviously that isn't a real solution.