setuptools: Clearly define django compatibility

setup.cfg: Clearly define the current django compatibility by limiting the version constraint to versions that are currently supported by upstream (at the time of writing 2.2, 3.1 and 3.2): https://www.djangoproject.com/download/

@maxking Can we please merge this?

added 4 commits

• 32d7af6e...db5a25ea - 3 commits from branch mailman:master
• e3cb572e - setuptools: Clearly define django compatibility

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mentioned in commit 16cf17ef

merged