Test: Narrow index variable typing #420

Job	Run time
Test mypyc suite with 32-bit Python	7m 21s
Test suite with py38-ubuntu, mypyc-compiled	18m 11s
Test suite with py38-windows-64	19m 45s
Test suite with py39-ubuntu	16m 57s
Test suite with py310-ubuntu	15m 32s
Test suite with py311-ubuntu, mypyc-compiled	16m 5s
Test suite with py312-ubuntu, mypyc-compiled	18m 29s
Test suite with py313-ubuntu, mypyc-compiled	18m 34s
mypyc runtime tests with py39-macos	6m 59s
Type check our own code (py38-ubuntu)	1m 58s
Type check our own code (py38-windows-64)	2m 31s
Formatting and code style with Black + ruff	33s
	2h 22m 55s

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