Tests

Enable warn_unreachable = True for `mypyc` and all other files as well #17375

Job	Run time
Test mypyc suite with 32-bit Python	9m 0s
Test suite with py310-ubuntu	10m 58s
Test suite with py311-ubuntu	8m 15s
Test suite with py312-ubuntu, mypyc-compiled	12m 42s
Test suite with py39-ubuntu, mypyc-compiled	14m 10s
Test suite with py313-windows-64	23m 6s
Test suite with py313-ubuntu, mypyc-compiled	13m 9s
Formatting and code style with Black + ruff	1m 49s
mypyc runtime tests with py39-macos	7m 39s
Test suite with py314-dev-ubuntu	15m 14s
Type check our own code (py39-windows-64)	2m 18s
Type check our own code (py39-ubuntu)	1m 36s
	1h 59m 56s

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