Drop Python 3.7 support and update CI envs

Merged Christoph Reiter requested to merge creiter/pygobject:drop-py37 into master

Python 3.7 will be EOL in 3 months, so before the next release.

  • Update the Python versions in the CI images
  • Bump the docker base image from focal to jammy to test against a newer stack. This triggers new deprecation warnings which we need to disable for now.
  • Bump the old docker based image to from buster to bullseye, moving from Python 3.7 to 3.9.

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