Better `tox` configuration (#15777) #10681

Job	Run time
Test mypyc suite with 32-bit Python	10m 10s
Test suite with py38-ubuntu, mypyc-compiled	21m 51s
Test suite with py38-windows-64	35m 34s
Test suite with py39-ubuntu	22m 38s
Test suite with py310-ubuntu	21m 32s
Test suite with py311-ubuntu, mypyc-compiled	22m 42s
Test suite with py312-ubuntu, mypyc-compiled	33m 42s
mypyc runtime tests with py38-macos	7m 4s
mypyc runtime tests with py38-debug-build-ubuntu	6m 41s
Type check our own code (py38-ubuntu)	2m 31s
Type check our own code (py38-windows-64)	2m 57s
Formatting and code style with Black + ruff	48s
	3h 8m 10s

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