Tests

Add --disallow-str-iteration flag #20105

Job	Run time
Test mypyc suite with 32-bit Python	9m 34s
Test suite with py311-ubuntu	9m 41s
Test suite with py314t-ubuntu, mypyc-compiled	17m 25s
mypyc runtime tests with py313-macos	6m 13s
Test suite with py314-ubuntu, mypyc-compiled	15m 46s
Parallel tests with py314-ubuntu, mypyc-compiled	27m 41s
Test suite with py314-windows-64	27m 4s
Type check our own code (py310-windows-64)	2m 24s
Parallel tests with py314-ubuntu, interpreted	23m 32s
Test suite with py313-ubuntu, mypyc-compiled	14m 45s
Test suite with py312-ubuntu	10m 9s
Type check our own code (py310-ubuntu)	1m 45s
Test suite with py310-ubuntu, mypyc-compiled	15m 57s
Formatting and code style with Black + ruff	2m 5s
mypyc runtime tests with py310-ubuntu	5m 36s
	3h 9m 37s

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