Tests

Document unreachability handling of `return NotImplemented` #20080

Job	Run time
Test mypyc suite with 32-bit Python	9m 32s
mypyc runtime tests with py310-ubuntu	5m 38s
Parallel tests with py314-ubuntu, interpreted	24m 14s
Type check our own code (py310-windows-64)	2m 17s
Test suite with py313-ubuntu, mypyc-compiled	14m 41s
Test suite with py311-ubuntu	9m 4s
Parallel tests with py314-ubuntu, mypyc-compiled	25m 57s
Type check our own code (py310-ubuntu)	1m 44s
Test suite with py312-ubuntu	10m 4s
mypyc runtime tests with py313-macos	6m 31s
Test suite with py314-ubuntu, mypyc-compiled	14m 49s
Test suite with py314-windows-64	27m 34s
Test suite with py310-ubuntu, mypyc-compiled	14m 56s
Test suite with py314t-ubuntu, mypyc-compiled	16m 54s
Formatting and code style with Black + ruff	1m 58s
	3h 5m 53s

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