Replace `tox` with `pytest` and `Makefile`, cleanup in `docs/Makefile… #1069

Job	Run time
run-test-suite (ubuntu-latest, 3.13, latest)	10m 19s
run-test-suite (ubuntu-latest, 3.9, 9.4)	10m 51s
run-test-suite (ubuntu-22.04, 3.7, 9.1)	10m 3s
run-test-suite (ubuntu-latest, 3.9, latest)	11m 20s
	42m 33s

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