Preserve AttributeError in slotted classes with cached_property

[dlax]

Summary

In slotted classes' generated __getattr__(), we try __getattribute__() before __getattr__(), if available, and eventually let AttributeError propagate. This matches better with the behaviour described in Python's documentation Customizing attribute access.

Fix #1230

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[hynek]

ping @diabolo-dan :)

[hynek]

thanks everyone!