TurboGears2 migrated to pytest and GitHub Actions

[amol-]

TurboGears2 was moved from Travis to GitHub Actions. During the change it was took the opportunity to also enable testing on more recent Python versions (3.9 and 3.10). To ease maintenance on more recent Python version, the testsuite was also migrated to PyTest.

Python 3.11 doesn't currently pass tests due to some incompatibilities in Kajiki. TurboGears2 itself works fine, but until Kajiki si fixed no release will be able to happen as Kajiki is the default template engine.