Tests

Add --disallow-str-iteration flag #20110

Job	Run time
Test mypyc suite with 32-bit Python	9m 31s
Test suite with py314-ubuntu, mypyc-compiled	14m 44s
Test suite with py314-windows-64	27m 27s
Test suite with py311-ubuntu	9m 42s
mypyc runtime tests with py310-ubuntu	5m 40s
Type check our own code (py310-ubuntu)	1m 42s
Test suite with py310-ubuntu, mypyc-compiled	15m 13s
Test suite with py314t-ubuntu, mypyc-compiled	17m 33s
Formatting and code style with Black + ruff	2m 4s
Parallel tests with py314-ubuntu, mypyc-compiled	30m 40s
mypyc runtime tests with py313-macos	5m 34s
Type check our own code (py310-windows-64)	2m 25s
Test suite with py312-ubuntu	9m 56s
Test suite with py313-ubuntu, mypyc-compiled	15m 12s
Parallel tests with py314-ubuntu, interpreted	22m 59s
	3h 10m 22s

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