Define __getattribute__() instead of __getattr__() on slotted classes with cached properties

[dlax]

Summary

Fix https://github.com/python-attrs/attrs/issues/1288

Method __getattribute__() is documented as the "way to to actually get total control over attribute access" so we change the implementation of slotted classes with cached properties by defining a __getattribute__() method instead of __getattr__() previously.

Just changing that preserves the current behaviour, according to the test suite, but also makes sub-classing work better, e.g. when the subclass is not an attr-class and also defines a custom __getattr__() as evidenced in added test from #1288.

In tests, we replace most custom __getattr__() definitions by equivalent __getattribute__() ones, except in regression tests where __getattr__() is explicitly involved. Also, in test_slots_with_multiple_cached_property_subclasses_works(), we replace the if hasattr(super(), "__getattr__"): by a try:/except AttributeError: as using hasattr(..., "__getattribute__") would be meaningless since __getattribute__() is always defined.

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