move utility code reference of __Pyx_Method_ClassMethod() into Entry to allow its eventual removal at a later stage

--HG--
extra : transplant_source : 4%93%F3HQ/%0C%D4%28%FB23%1E%F6%FE%E1%D8%8AT%EB

--HG--
extra : transplant_source : %5D%18L%A0%A91/%B1%96Z%9ECr%07%AD%C7wp%7B%CA

Better support for PyPy's cpyext C-API

rework of optional kw-only arg unpacking

warn when taking char* from a global Python variable

replace plain '#define CYTHON_PEP393_ENABLED' by flag #define as 1 or 0 to make it usable from Cython code

--HG--
extra : transplant_source : %98%A3s%16%85%D7%5C%18%AC%C4%EFe%A624ZcU%EC%A8

--HG--
extra : transplant_source : %9A%B9%B3%3B%3E%07%C1%F5H%3F%AFm%F1t%B9%3Cg4%C9/

replace plain '#define CYTHON_PEP393_ENABLED' by flag #define as 1 or 0 to make it usable from Cython code