Make is_sub_path faster #13574

Job	Run time
Test suite with py38-ubuntu, mypyc-compiled	18m 9s
Test suite with py38-windows-64	20m 41s
Test suite with py39-ubuntu	16m 26s
Test suite with py310-ubuntu	16m 18s
Test mypyc suite with 32-bit Python	7m 12s
Test suite with py311-ubuntu, mypyc-compiled	15m 46s
Test suite with py312-ubuntu, mypyc-compiled	17m 50s
Test suite with py313-ubuntu, mypyc-compiled	18m 29s
mypyc runtime tests with py39-macos	4m 36s
Type check our own code (py38-ubuntu)	1m 54s
Type check our own code (py38-windows-64)	2m 17s
Formatting and code style with Black + ruff	33s
	2h 20m 11s

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