Tests

Add --disallow-str-iteration flag #20267

Job	Run time
Test mypyc suite with 32-bit Python	1m 45s
Test suite with py314-ubuntu, mypyc-compiled	1m 55s
Test suite with py314t-ubuntu, mypyc-compiled	1m 55s
Test suite with py312-ubuntu	1m 56s
Test suite with py314-windows-64	1m 48s
Test suite with py310-ubuntu, mypyc-compiled	1m 55s
mypyc runtime tests with py313-macos	2m 1s
mypyc runtime tests with py310-ubuntu	1m 58s
Parallel tests with py314-ubuntu, mypyc-compiled	1m 55s
Test suite with py313-ubuntu, mypyc-compiled	1m 55s
Formatting and code style with Black + ruff	1m 57s
Test suite with py311-ubuntu	1m 55s
Type check our own code (py310-ubuntu)	1m 45s
Parallel tests with py314-ubuntu, interpreted	1m 56s
Type check our own code (py310-windows-64)	1m 49s
	28m 25s

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