New failures in test_all stubs tests

[mdickinson]

Details of the failure:

======================================================================
FAIL: test_all (traits.stubs_tests.test_all.TestAnnotations.test_all) (file_path=PosixPath('/Users/mdickinson/.venvs/traits/lib/python3.13/site-packages/traits/stubs_tests/examples/Any.py'))
Run mypy for all files contained in traits.stubs_tests/examples
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/mdickinson/.venvs/traits/lib/python3.13/site-packages/traits/stubs_tests/test_all.py", line 44, in test_all
    self.assertRaisesMypyError(file_path)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^
  File "/Users/mdickinson/.venvs/traits/lib/python3.13/site-packages/traits/stubs_tests/util.py", line 178, in assertRaisesMypyError
    raise AssertionError(
    ...<3 lines>...
    )
AssertionError: expected errors on lines 58, but no mypy errors were reported for those lines

I'm guessing that this is related to a change in mypy version.

[mdickinson]

The previous error (e.g., with mypy 1.15.0) was:

../traits/stubs_tests/examples/Any.py:58: error: Incompatible types in assignment (expression has type "Instance[Foo | None]", base class "Superclass" defined the type as "traits.trait_types.Any")  [assignment]
Found 44 errors in 2 files (checked 1 source file)

With mypy 1.16.0, this error is no longer present.

This appears to be due to changes in descriptor handling in mypy 1.16.0 (e.g., python/mypy#18831): mypy is now clever enough to understand that the Any() trait allows any value. It's not clear to me that this is really the behaviour we want, but it's not obviously wrong.