setup.py: Force Sphinx > 2 for readthedocs
FIX
readthedocs uses an old version of sphinx for projects created before 2020: https://github.com/mgeier/sphinx-last-updated-by-git/issues/1
So force a newer version.
FIX
readthedocs uses an old version of sphinx for projects created before 2020: https://github.com/mgeier/sphinx-last-updated-by-git/issues/1
So force a newer version.