TST: Bump mypy from 0.931 to 0.940

[charris]

Backport of #21180.

Use the new mypy 0.940 release when running tests.

There are two notable changes that require an update of the test suite:

• The repr from homogenous tuples was changed from tuple[Any] to tuple[Any, ...].
• iter can now return actual Iterator[Any] subclasses, rather than just subtypes.

[charris]

@BvB93 There is a small problem with the backport:

path = '/home/vsts/work/1/s/build/testenv/lib/python3.8/site-packages/numpy/typing/tests/data/reveal/fromnumeric.pyi'
expression = 'reveal_type(np.ravel(b))', reveal = 'ndarray[Any, dtype[bool_]]'
expected_reveal = '106: note: Revealed type is "numpy.ndarray[Any, Any]"'
lineno = 106