[Cython] CEP 1001 - Custom PyTypeObject extensions

[Dag Sverre Seljebotn]

This comes from a refactor of the work on CEP 1000: A PEP proposal, with 
a hack for use in current Python versions and in the case of PEP 
rejection, that allows 3rd party libraries to agree on extensions to 
PyTypeObject.

http://wiki.cython.org/enhancements/cep1001

If this makes it as a PEP, I don't think we need to think about having 
CEP 1000 accepted as a PEP.

Comments?

Dag