Re-enable mypy and sync mypy.ini from skeleton #5648

Job	Run time
collateral (diffcov)	5m 28s
test (3.8, ubuntu-latest)	4m 2s
test_cygwin (39, windows-latest)	14m 9s
collateral (docs)	1m 51s
test (3.8, windows-latest)	11m 33s
test (3.13, ubuntu-latest)	5m 55s
test (3.13, macos-latest)	4m 26s
test (3.13, windows-latest)	11m 4s
test (3.9, ubuntu-latest)	4m 10s
test (3.10, ubuntu-latest)	4m 3s
test (3.11, ubuntu-latest)	4m 5s
test (3.12, ubuntu-latest)	5m 27s
test (3.14, ubuntu-latest)	6m 3s
test (pypy3.10, ubuntu-latest, stdlib)	15m 36s
test (ubuntu-latest, 3.10, stdlib)	4m 6s
test (3.8, macos-13)	6m 45s
integration-test	0s
check	2s
release	0s
	1h 48m 45s

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