bpo-9566 & bpo-30747: Silence warnings from pyatomic.h macros (#3140)

* bpo-9566: Silence warnings from pyatomic.h macros

Apparently MSVC is too stupid to understand that the alternate branch is
not taken and emits a warning for it.

Warnings added in https://github.com/python/cpython/pull/2383

* bpo-9566: A better fix for the pyatomic.h warning

* bpo-9566: Remove a slash