[Cython] PEP 487 -- Simpler customisation of class creation

[Stefan Behnel]

Hi all!

Here's a nice PEP that we should implement in Cython. It has been accepted
for inclusion in Python 3.6.

https://www.python.org/dev/peps/pep-0487/

We should at least support it for all Cython implemented Python classes.
Backporting it to older CPython versions would then require an internally
defined metaclass, since Python itself would otherwise not support it for
subclasses.

Any volunteers? Please reply on the cython-devel mailing list.

Stefan