[pre-commit.ci] pre-commit autoupdate

[pre-commit-ci[bot]]

updates:

• github.com/asottile/pyupgrade: v2.26.0 → v2.28.0
• github.com/mgedmin/check-manifest: 0.46 → 0.47

[psi29a]

@graingert @GrayAn I'm tidying this PR up and fixing some cruft... might not be 100%, but it gets us further long in proper CI.

• fixed tox (using latest version)
• fixed a few setup.cfg/pyproject issues
• fixed readthedocs

[psi29a]

Finally got something working... but now we're waiting on codecov?

@graingert and @GrayAn , you know anything?