Add trove classifier for Python 3.9

[D3X]

Pull Request check-list

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• Does $ tox pass with this change (including linting)?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?

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Description of change

Adds a trove classifier for Python 3.9. That version was added to the list of tox envs in cb97b9b

[chayim]

Thanks for your contribution! I had this committed in another PR as part of #1524 but I'm glad someone has broken it out into a small change. Merging.