Remove pin to sphinx 5.x and remove sphinx-asyncio. Sphinx now supports
asyncio out of the box. Everything I've checked looks fine although I
haven't looked at every page.
Fixes#3239
Sphinx is pinned to <6 because of a conflict with sphinx_rtd_theme
Tox is pinned to <4 because we're affected by some backwards-incompatible
renamings in the config file.
This division was just complicating things unnecessarily.
Also adopt pip-tools instead of doing it all by hand.
No pinned versions have been changed in this commit.
Ben Darnell