Drop Python 2 support

[itamarst]

Fixes https://tahoe-lafs.org/trac/tahoe-lafs/ticket/3873
Fixes https://tahoe-lafs.org/trac/tahoe-lafs/ticket/3327

• CircleCI has no Python 2
• GitHub Actions has no Python 2
• Modern Fedora
• Oracle Linux as replacement for CentOS, since the latter is dead
• Switch image building back to schedule
• Everything passes
• Update setup.py (metadata plus minimum version)
• Update README
• REVIEWER: Anything else that needs updating?
• REVIEWER: Remove old checks from requirements for branch (needs to be done by @meejah or @exarkun since I lack permissions)

https://www.youtube.com/watch?v=aiwA0JrGfjA

[coveralls]

Coverage decreased (-1.1%) to 94.546% when pulling 5647b4a on 3873-welcome-to-the-world-of-tomorrow into 296bc3e on master.

[exarkun]

Thanks. I left some comments inline.

[exarkun]

Are there any best practices for when to include minor version numbers too?

[itamarst]

🤷 not sure anyone actually looks at these anymore, the python_requires is the important bit.

[itamarst]

Not sure what's up on macOS. Will have to spin up tahoe dev environment on my macOS VM I guess :(

[exarkun]

I am now looking at Settings > Branches > Branch Protection Rules > master > Edit > Require status checks to pass before merging and I am clicking the x next to:

• ci/circleci: centos-8 (centos is gone, hello oraclelinux)
• ci/circleci: debian-9 (debian 9 is gone, hello debian 11)
• ci/circleci: fedora-28 (fedora 28 is gone, hello fedora 35)
• ci/circleci: fedora-29 (fedora 29 is gone, hello fedora 35)
• ci/circleci: lint (lint is gone, hello codechecks)
• ci/circleci: python37 (it's all python3 now)
• ci/circleci: ubuntu-16-04 (ubuntu 16.04 is gone, hello 20.04)
• coverage (macos-10.15, 2.7) (python2.7 is gone, hello coverage (macos-latest, 3.9))
• coverage (windows-latest, 2.7) (python2.7 is gone, hello coverage (windows-latest, 3.7, 3.8. 3.9))
• integration (macos-10.15, 2.7) (python2.7 is gone, hello integration (macos-latest, 3.9))
• integration (windows-latest, 2.7) (python2.7 is gone, hello integration (windows-latest, 3.7, 3.9))
• packaging (macos-10.15, 2.7) (python2.7 is gone, hello packaging (macos-10.15, 3.9))
• packaging (ubuntu-latest, 2.7) (python2.7 is gone, hello packaging (ubuntu-latest, 3.9))
• packaging (windows-latest, 2.7) (python2.7 is gone, hello packaging (windows-latest, 3.9))

These replacements mostly seem pretty good. One thing that stands out looking at this list is that we're not very consistent in whether we pick macos-latest or macos-10.15. We used to pick macos-10.15 everywhere (even though we pick windows-latest and ubuntu-latest). Now we use both macos-10.15 and macos-latest. Maybe that's as it must be to get the necessary versions of Python? But from https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners I see that macos-11 should currently be the same as macos-latest - maybe we should prefer macos-11 to avoid getting shifted to a newer version whenever github feels like it? Of course this same reasoning could apply to the Windows and Ubuntu images... However, I can't say I'm familiar enough with GitHub's image update process to know if this is a real issue or not. Probably makes sense to leave this as-is until we know a reason to do it differently, unless you already do...

And now I have clicked the "Save changes" button.

[exarkun]

Thanks!