mypy

Build mpdecimal from source to workaround unreliable launchpad.net (#… #21006

Job	Run time
Run mypy on Lib/test/libregrtest	23s
Run mypy on Lib/tomllib	15s
Run mypy on Tools/cases_generator	19s
Run mypy on Tools/build	16s
Run mypy on Tools/jit	19s
Run mypy on Tools/clinic	20s
Run mypy on Tools/check-c-api-docs	16s
Run mypy on Lib/_pyrepl	18s
Run mypy on Tools/peg_generator	18s
	2m 44s

Provide feedback