Add a bunch of new python versions (and pypy) to the CI

[masklinn]

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[masklinn]

• item: could probably stop installing tox until used for testing again
• item: cpython 3.10 is much slower than 3.9, according to unittest's reporting it needs 25s to run the tests (on github's VMs) compared to 11, it's also much slower at installing dependencies (13s versus 5s)
• item: pypy is even slower at both, it needs 40s to install the dependencies and 60 seconds to run the tests

There's some variability, but the range of performance seems reliable: all CI before 3.10 occur in 25~30s, 3.10 and up need a minute, and pypy needs 2. That's really not great.

[masklinn]

Really cool stuff: locally, 3.10 runs the tests in half the time 3.8 does (6.5s versus 14), and pypy works at about the same thing (as 3.10, so 6.5s).

This is not just a case of github's runners being slow, this is github runners displaying performance behaviour opposite that of local. That doesn't seem like something justified by the two architectures not matching. And time -l reports the test run under 3.10 having a peak memory footprint (and maximum RSS) 30% lower than 3.8, and both are in the hundred megs, so doesn't seem likely to be a swapping thing (if github's workers even have swap)