Drop support for Python 3.9

Python 3.9 is reaching the end of its life tomorrow. We should also drop support from mypy at some point, though not necessarily immediately.

Concrete issue:

	# TODO: macos-13 is the last one to support Python 3.9, change it to macos-latest when updating the Python version
	os: macos-13

currently uses a macos-13 runner, which GitHub is working to deprecate. Therefore, this job may start failing in the next month or so. We could drop this one job and still keep best-effort 3.9 support for some time, or we could just use this as a reason to drop 3.9 for now. (Thanks @hugovk for pointing me to this.)

Please write here if support for Python 3.9 is still important to you.