Tests

Add --disallow-str-iteration flag #20270

Job	Run time
Test mypyc suite with 32-bit Python	9m 19s
Test suite with py310-ubuntu, mypyc-compiled	15m 9s
Test suite with py313-ubuntu, mypyc-compiled	15m 24s
Test suite with py312-ubuntu	9m 53s
Test suite with py311-ubuntu	9m 42s
Test suite with py314t-ubuntu, mypyc-compiled	17m 28s
Test suite with py314-ubuntu, mypyc-compiled	14m 51s
mypyc runtime tests with py313-macos	6m 37s
Parallel tests with py314-ubuntu, interpreted	25m 5s
Test suite with py314-windows-64	27m 41s
Parallel tests with py314-ubuntu, mypyc-compiled	29m 4s
Type check our own code (py310-ubuntu)	1m 44s
mypyc runtime tests with py310-ubuntu	5m 36s
Formatting and code style with Black + ruff	1m 56s
Type check our own code (py310-windows-64)	2m 34s
	3h 12m 3s

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