Update Python support matrix

[stephenfin]

Update the Python support matrix, dropping Python 3.6 support and adding 3.9, 3.10 and 3.11.

To do this, we must also update a few other things, such as the pre-commit plugins. The list of changes is thus:

• tox: Replace '--strict' with '--strict-markers'
• Use collections.abc, not collections
• pre-commit: Bump plugin versions
• Add support for recent Python versions
• Use latest Python + Action versions in CI

[stephenfin]

@ikalnytskyi I'm going to cross my fingers and hope you're okay with me merging this to get the CI passing again :crossed_fingers: If you're not, please feel free to revert it and accept my apologies for interfering.