mypy

GH-139040: Add Dev Container instructions in JIT README (#139041) #9624

Job	Run time
Run mypy on Lib/tomllib	16s
Run mypy on Lib/test/libregrtest	20s
Run mypy on Tools/jit	22s
Run mypy on Tools/cases_generator	19s
Run mypy on Tools/clinic	20s
Run mypy on Tools/build	23s
Run mypy on Lib/_pyrepl	20s
Run mypy on Tools/peg_generator	20s
	2m 40s

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