support mock.patch.object on HasTraits

[minrk]

mock.patch.object requires allowing delattr(has_traits, "trait") to work and hasattr(has_traits, "trait") to be False afterward.

This fixes mock.patch.object on HasTraits objects, with a previously failing test.

Without this patch, test fails with:

    def __exit__(self, *exc_info):
        """Undo the patch."""
        if not self.is_started:
            return

        if self.is_local and self.temp_original is not DEFAULT:
            setattr(self.target, self.attribute, self.temp_original)
        else:
>           delattr(self.target, self.attribute)
E           AttributeError: __delete__

Initially, I had it just clear the state without the sentinel (could be useful for restoring defaults, for example), but in that case hasattr(obj, 'trait') returns True and mock.patch doesn't restore the before-patch value, it always returns to the default state.

I think the logic in unittest.mock is wrong in how it decides whether to set the original value back, but this is what we need to work with the current standard library.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.20%. Comparing base (5b398be) to head (f948af7).

Components	Coverage Δ
traitlets	85.62% <100.00%> (+0.01%)	⬆️
tests	99.07% <100.00%> (+<0.01%)	⬆️

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