mypy

gh-142119: Clarify that one `contextvars.Token` can only reset once i… #19288

Job	Run time
Run mypy on Tools/check-c-api-docs	21s
Run mypy on Tools/cases_generator	19s
Run mypy on Lib/test/libregrtest	27s
Run mypy on Lib/tomllib	21s
Run mypy on Tools/build	18s
Run mypy on Tools/clinic	20s
Run mypy on Lib/_pyrepl	22s
Run mypy on Tools/peg_generator	23s
Run mypy on Tools/jit	18s
	3m 9s

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