This updates the classifiers for the package to list Python 3.9 and 3.10

support, and to include metadata that will help ensure pip installs

the right package based on the current version of Python in the future,

which will matter as we end up dropping older versions down the road.
The tox configuration has also been updated to test Python 3.10, and to

use compatible release clauses for the right versions of Django.

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Tox successfully ran all unit tests.