Python 3.8 has reached EOL

[hacklschorsch]

Python 3.8 is EOL - we should remove it from our code (test suites etc).

See ​https://devguide.python.org/versions/

Like ticket:4130 but for everything.

[btlogy]

I had prepared a first PRs:

• ​Remove Python 3.8 and OracleLinux 8 with it tahoe-lafs#1405

But strong of having successfully tested Python 3.9 on Oracalelinux 8 (see ticket:4135#comment:3), I'm in favor of this new PR:

• ​Remove Python 3.8 EOL and unit test on Python 3.9 instead tahoe-lafs#1406

[btlogy]

While working on ​tahoe-lafs#1406 and ticket:3541#comment:7, I've discovered that:

• only GH maintainers can trigger the build of the docker images
• only GH maintainers can consume the secrets in dockerhub-auth context (to pull of push), because those are only available for branches created within the repo ("Pass secrets to builds from forked pull requests" is disabled in CircleCI).

[hacklschorsch]

Some of this has been addressed in ​https://github.com/tahoe-lafs/tahoe-lafs/commit/53451abef48c9ed72bf4a839fceb63b1284557b2

[btlogy]

This ticket was pending at least partly of ticket:4143 for this new oraclelinux-8 image updated with Python 3.9.

This image has already been published (because this workflow is unfortunately not automated when merging the master), so the ​PR#1406 is already showing a green ​check for this builder.

However, the CircleCI config in this PR is referring to an image which will be defined in the master only when/if ​PR#1409 will be merged.

This feels a bit messy, but that's how things are actually setup today. I hope we can improve this in ticket:3484.

[btlogy]

Since I've prepared the comment above, ​PR#1406 has been rejected in favor of ​PR#1418 which does not cover the part where we were reviving the oraclelinux-8 and fedora-35 builders...

I'll propose to bring those back separately in the context of ticket:4143.

For the rest of this issue: support for Python 3.8 should have been properly removed by now.