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Here's an updated patch with several improvements.  First,
the name is now GetSetType and isgetset().  Also, I've added
support for member descriptors with MemberDescriptorType and
ismemberdescriptor().  I've also added pydoc help for member
descriptors.  The appropriate things are done for
implementations other than CPython that don't have such
extension types.

This patch includes the usual docs and tests updates.

The other improvement is that rather than relying on
accidents of implementation in the C Python interpreter,
I've added a built-in _types module which implements
specifically the types that are needed.  This seems like the
best way to ensure that we have accurate types without tying
down future development of obscure other modules.

This may make it more difficult to backport to Python 2.4,
but that's a different concern.  Better to DTRT for Python
2.5 first.

Assigning back to Georg for another quick review, but feel
free to assign it back to me for final checkin (assuming
Anthony okays it).