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Looks fine, I noticed a few things:
- perhaps GetSet should be named GetSetDescriptor for
consistency with DataDescriptor and MemberDescriptor
- in pydoc, shouldn't "data descriptor" be tested with
"inspectisdatadescriptor"?
- in the docs, false should be False
- test_twelve should now be called test_thirteen ;) and the
comment in it should be adapted
- is the test suite expected to pass in implementations
without the newly tested descriptor types?