Support Python 3.10

[simonw]

I started work on this in #1481 where I found a Python 3.10 bug that needs a workaround in Janus, see:

• Error with Python 3.10: "ValueError: loop argument must agree with lock" aio-libs/janus#358

This is a tracking issue for anything else that shows up.

This is also needed for the Homebrew package to upgrade to 3.10:

• datasette: migrate to python@3.10 Homebrew/homebrew-core#86932

[simonw]

I think that SQLite error message difference was caused by python/cpython@a50e283 or related work.

[simonw]

Janus 0.6.2 is out now and should have the fix.