fixes for unittest.runner

[tungol]

I have a MR for stubtest out at python/mypy#18251. One of the things that got flagged was unittest.runner.TextTestRunner.resultclass and I was having a hard time figuring it out. I made a version of unittest/runner.py locally that had all of our existing stub annotations inline with the actual code, and ran mypy on that to try and figure it out. This is various fixes that were needed to make mypy satisfied with that version of the file.

• Updating _ResultClassType to use TextTestResult[Any]: This was the primary issue that got flagged in my MR for stubtest.
• _WritelnDecorator takes _SupportsWriteAndFlush as the argument. It is _TextTestStream, which adds writeln on top of that, but it doesn't need to be passed something that already has writeln.
• writeln doesn't actually return anything
• durations is an integer which indexes into unittest.result._DurationsType, not unittest.result._DurationsType itself. It's TestResult.collectedDurations that is _DurationsType.
• TextTestRunner.warnings is passed to warnings.simplefilter, which is typed for literals.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉