Better `tox` configuration #10676

Job	Run time
Test mypyc suite with 32-bit Python	14m 28s
Test suite with py38-ubuntu, mypyc-compiled	21m 29s
Test suite with py38-windows-64	34m 18s
Test suite with py39-ubuntu	22m 45s
Test suite with py310-ubuntu	22m 11s
Test suite with py311-ubuntu, mypyc-compiled	23m 49s
Test suite with py312-ubuntu, mypyc-compiled	23m 36s
mypyc runtime tests with py38-macos	9m 34s
mypyc runtime tests with py38-debug-build-ubuntu	6m 37s
Type check our own code (py38-ubuntu)	2m 33s
Type check our own code (py38-windows-64)	3m 25s
Formatting and code style with Black + ruff	39s
	3h 5m 24s

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