Tests

Add --disallow-str-iteration flag #20138

Job	Run time
Test mypyc suite with 32-bit Python	9m 30s
Test suite with py310-ubuntu, mypyc-compiled	1m 11s
Test suite with py314t-ubuntu, mypyc-compiled	1m 22s
mypyc runtime tests with py310-ubuntu	5m 27s
Test suite with py313-ubuntu, mypyc-compiled	1m 5s
Type check our own code (py310-ubuntu)	1m 31s
Parallel tests with py314-ubuntu, mypyc-compiled	1m 4s
mypyc runtime tests with py313-macos	4m 15s
Test suite with py314-ubuntu, mypyc-compiled	1m 6s
Parallel tests with py314-ubuntu, interpreted	24m 8s
Test suite with py311-ubuntu	9m 17s
Test suite with py312-ubuntu	10m 1s
Test suite with py314-windows-64	27m 26s
Type check our own code (py310-windows-64)	2m 36s
Formatting and code style with Black + ruff	2m 1s
	1h 42m 0s

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