Build CPython 3.11 by default (no longer behind prerelease_pythons)

[minrk]

not to merge before #1223, which adds 3.11.0rc1.

closes #1221

[joerick]

Thanks @minrk! I should get a release out with this today.

[minrk]

Awesome. I'm excited to start publishing my wheels! I love cibuildwheel so much.

[minrk]

I don't understand the failure on Windows or how it could be related to this PR. The test is running with Python 3.7 which didn't change and the error message is claiming that pip 20.0.2 conflicts with itself?

ERROR:root:failed to download pip version 20.0.2, pip download exit code 1
The conflict is caused by:
    The user requested pip==20.0.2
    The user requested (constraint) pip==20.0.2

Maybe an intermittent error?

[joerick]

I don't understand it either. Perhaps a failed download caused a bad error message. I've restarted it, lets see...

[Czaki]

I do not see full message, but I meet similar things, then are problems with pypi and some version was not reported as available.

[henryiii]

We've been seeing that sort of message a lot recently. But restarting it usually fixes it, so I've assumed it's a flaky PyPI issue.