mypy

gh-77328: Update `base64` module RFC references to RFC 4648 (#151275) #21874

Job	Run time
Run mypy on Lib/_pyrepl	18s
Run mypy on Tools/peg_generator	16s
Run mypy on Tools/check-c-api-docs	13s
Run mypy on Lib/test/libregrtest	20s
Run mypy on Tools/build	16s
Run mypy on Tools/clinic	14s
Run mypy on Lib/tomllib	19s
Run mypy on Tools/cases_generator	21s
Run mypy on Tools/jit	14s
	2m 31s

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