Tests

Add --disallow-str-iteration flag #20129

Job	Run time
Test mypyc suite with 32-bit Python	9m 52s
Test suite with py310-ubuntu, mypyc-compiled	15m 31s
Test suite with py313-ubuntu, mypyc-compiled	14m 20s
Test suite with py312-ubuntu	10m 13s
mypyc runtime tests with py313-macos	4m 27s
Type check our own code (py310-ubuntu)	1m 44s
Test suite with py311-ubuntu	9m 14s
mypyc runtime tests with py310-ubuntu	5m 36s
Test suite with py314t-ubuntu, mypyc-compiled	17m 36s
Test suite with py314-windows-64	27m 30s
Formatting and code style with Black + ruff	1m 56s
Type check our own code (py310-windows-64)	2m 19s
Parallel tests with py314-ubuntu, mypyc-compiled	28m 4s
Parallel tests with py314-ubuntu, interpreted	24m 47s
Test suite with py314-ubuntu, mypyc-compiled	15m 10s
	3h 8m 19s

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