Test on Python 3.9 and 3.10

[adamchainz]

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• Add classifier tags
• Add tox test environments
• Simplify tox envlist syntax
• Move from python setup.py test to directly invoking unittest, to avoid this warning from setuptools:

WARNING: Testing via this command is deprecated and will be removed in a future version. Users looking for a generic test entry point independent of test runner are encouraged to use tox.