Add support for Python 3.10

[hugovk]

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Fixes #1006 🦕

[plamut]

@hugovk Thank you for the PR, but before we can claim Python 3.10 support, the test infrastructure needs to be set up first to run the CI checks under Python 3.10 (#1006 (comment)).

It is not guaranteed that the library works with Python 3.10 out of the box, because all dependencies need to support it as well, which may take some time.

[hugovk]

Thanks, I saw that comment but it's a bit light on detail. Is it something you need to deal with internally? Feel free to close this PR if it's premature!

[plamut]

@hugovk Indeed, it's an internal thing to set up the CI checks. If it was just updating the python_requires setting and the classifiers, it would probably have already been done. 🙂

Closing this for now until all prerequisites are in place and all checks pass under Python 3.10.