bpo-35177, Python-ast.h: Fix compiler warning on Yield

[vstinner]

Partially revert commit 5f2df88:
remove "#undef Yield" from Python-ast.h and move it back to .c files.

https://bugs.python.org/issue35177

[vstinner]

Fix this warning:

winbase.h(102): warning C4005: 'Yield': macro redefinition                                                         

[vstinner]

I checked on my Windows VM and an AppVeyor that warnings are gone.

[zooba]

Aren't most of these redundant now that python-ast.h undefines it?

[vstinner]

I made the same remark than you, but... no :-) <winbase.h> and "Python-ast.h" both define Yield. Usually, we don't care of Yield from Python-ast.h, so we undefine it after including Python-ast.h. Sometimes, <winbase.h> is included before "Python-ast.h" and so "Python-ast.h" has to undefine it.

Yield from Python-ast.h is only used once, in ast.c... Maybe Python-ast.h must never define it and ast.c should define it instead? It's a very stupid define...

#define Yield(a0, a1, a2, a3) _Py_Yield(a0, a1, a2, a3)

This PR restores the status quo of Python 3.7, since my previous attempt supposed to remove warnings introduced the "Yield warning".

[vstinner]

@zooba: If you see a smarter solution and/or care of this #undef, please go ahead :-) In the meanwhile, I restored the code as it was previously :-)