Add tests against Python 3.9 and claim support for it in our packages

[bmw]

We should at least do this once it is released which the release schedule expects to be October 5th.

As of writing this, Azure Pipelines does not offer a prerelease version of Python 3.9. You can check the included software on each OS in Azure at https://docs.microsoft.com/en-us/azure/devops/pipelines/agents/hosted?view=azure-devops&tabs=yaml#software.

[FelixSchwarz]

Just wanted to mention that Fedora rawhide has 3.9 already and the test suite passes with that version. :-)

[bmw]

This should be possible once actions/runner-images#1740 is resolved.

[bmw]

This should be unblocked because Python 3.9 should be available in the Linux images on Azure Pipelines.

[bmw]

I made quite a bit of progress on this since I started working on it on Wednesday. What I currently have is in the test-py39 branch which builds on #8401. You can see all tests passing besides mypy at https://dev.azure.com/certbot/certbot/_build/results?buildId=2893&view=results.

There's more work to be done though. I had to update dnspython to 2.0+ since previous versions do not support Python 3.9 which created new work for us. I think we need to:

• Fix Remove python_version setting from mypy.ini #7806. Newer versions of dnspython uses f-strings and we configure mypy to parse files as Python 2 code which doesn't support f-strings causing it to fail. I'd recommend doing this in another PR.
• Fix Integration tests for certbot-dns-rfc2136 #8433 and run tests to make sure everything still works. Since we're upgrading to a new major version of dnspython with some (what appear to be quite minor) API changes, I think we should manually run integration tests with certbot-dns-rfc2136 and the new version of dnspython to make sure it still works before we pin that version in our snaps and Docker images.

I think the most useful thing someone else could do is review #8401, however, if someone wants to work on these pieces, feel free to do so. I encourage people to update the issue if they do so we know who is doing what.