Re-enable mypy and sync mypy.ini from skeleton #5594

Job	Run time
collateral (diffcov)	5m 23s
test_cygwin (39, windows-latest)	15m 56s
collateral (docs)	1m 48s
test (3.8, ubuntu-latest)	3m 52s
test (3.8, windows-latest)	3m 55s
test (3.13, ubuntu-latest)	4m 5s
test (3.13, macos-latest)	4m 6s
test (3.13, windows-latest)	3m 58s
test (3.9, ubuntu-latest)	3m 58s
test (3.10, ubuntu-latest)	3m 51s
test (3.11, ubuntu-latest)	3m 53s
test (3.12, ubuntu-latest)	4m 5s
test (3.14, ubuntu-latest)	4m 5s
test (pypy3.10, ubuntu-latest, stdlib)	4m 4s
test (ubuntu-latest, 3.10, stdlib)	3m 47s
test (3.8, macos-13)	4m 6s
integration-test	0s
check	3s
release	0s
	1h 14m 55s

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