Bump minimum Python version requirement to 3.9 and update CI #2291

Job	Run time
static_checks	41s
link_checker	7s
build_and_test_py_standalone	2m 22s
build_and_test_nest_compartmental (master)	20m 41s
build_and_test_nest_desktop (master)	12m 42s
build_and_test_nest (v2.20.2)	29m 10s
build_and_test_nest (v3.0)	23m 1s
build_and_test_nest (v3.8)	24m 35s
build_and_test_nest (master)	50m 56s
	2h 44m 15s

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