[Cython] Call for help: official C-API 2 to 3 porting guide

[Christian Heimes]

Hello fellow developers!

Benjamin is working on a C-API 2 to 3 porting guide for the official 
Python docs. I know that you - the Cython developers - have spent lots 
of time on making Cython compatible with 2.3+ as well as 3.0. Could you 
please assist Benjamin with his task and give him some hints? We are 
especially interested in a list of gotchas and tricks how to write cross 
version compatible C code.

Thanks in advance!

Christian