Tests

Add --disallow-str-iteration flag #20143

Job	Run time
Test mypyc suite with 32-bit Python	10m 3s
Test suite with py310-ubuntu, mypyc-compiled	1m 16s
Type check our own code (py310-ubuntu)	1m 32s
Test suite with py314-windows-64	28m 31s
Parallel tests with py314-ubuntu, mypyc-compiled	1m 6s
mypyc runtime tests with py313-macos	6m 2s
Parallel tests with py314-ubuntu, interpreted	23m 55s
Formatting and code style with Black + ruff	1m 56s
Test suite with py311-ubuntu	9m 15s
Test suite with py313-ubuntu, mypyc-compiled	1m 3s
mypyc runtime tests with py310-ubuntu	5m 39s
Test suite with py312-ubuntu	10m 9s
Test suite with py314t-ubuntu, mypyc-compiled	1m 19s
Type check our own code (py310-windows-64)	2m 32s
Test suite with py314-ubuntu, mypyc-compiled	1m 5s
	1h 45m 23s

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