Tests

Add --disallow-str-iteration flag #20268

Job	Run time
Test mypyc suite with 32-bit Python	40s
Test suite with py314t-ubuntu, mypyc-compiled	50s
Test suite with py310-ubuntu, mypyc-compiled	49s
Test suite with py314-windows-64	42s
Parallel tests with py314-ubuntu, mypyc-compiled	50s
Test suite with py312-ubuntu	50s
mypyc runtime tests with py313-macos	53s
Test suite with py311-ubuntu	50s
Type check our own code (py310-ubuntu)	53s
mypyc runtime tests with py310-ubuntu	52s
Test suite with py313-ubuntu, mypyc-compiled	50s
Parallel tests with py314-ubuntu, interpreted	51s
Formatting and code style with Black + ruff	53s
Test suite with py314-ubuntu, mypyc-compiled	50s
Type check our own code (py310-windows-64)	42s
	12m 15s

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