Drop Python 3.6 build and add Python 3.11

[Owen Lamont]

I noticed a build for Python 3.6 failed here. Since Python 3.6 is past end of life I think it is reasonable to drop it. I've also tested git-of-theseus manually with no issues on Python 3.11 if we wanted to add that.

[Erik Bernhardsson]

Yeah that would make sense! I think it's as easy as just updating https://github.com/erikbern/git-of-theseus/blob/master/.github/workflows/ci-cd.yml