Update to Python 3.10

[tomach]

Summary of changes

Thanks a lot for your work @amotl!
I cherry-picked your commit from GH-675 and updated mypy to a newer version - not the latest one though. I thought a gradual approach might be better to keep things smoother and tackle changes incrementally without adding too much all at once.
So I would leave GH-654 open for now and address it once we tackle the upgrade to Python 3.11. wdyt?

Checklist

• Link to issue this PR refers to: https://github.com/crate/cloud/issues/2314
• Relevant changes are reflected in CHANGES.rst
• Added or changed code is covered by tests
• Documentation has been updated if necessary
• Changed code does not contain any breaking changes (or this is a major version change)

[plaharanne]

It looks good to me but I don't mind if @amotl gives his feedback too.

[amotl]

Every update helps. Thanks. In general, the gradual approach to keep things smoother dearly needs some more velocity, because newer versions of Python provide subtle but great advantages.