TODO: add a NEWS entry for all these changes.

Copy of Nick Coghlan's comment:
https://github.com/python/cpython/pull/10275#issuecomment-439339982

@vstinner For folks consuming source archives (Linux distros, anyone embedding Python in a larger application, etc) rather than prebuilt binaries, the build process is something that can break, even for Py_BUILD_CORE only changes.

For those folks, when they pull a new source archive, their builds may break, especially if they're applying additional downstream patches the way Linux distros do.

The NEWS entry for this header file rearrangement should go in the Build section (https://github.com/python/cpython/tree/master/Misc/NEWS.d/next/Build) rather than the C API section (since these headers aren't part of the public API at all), but it should still exist.