Previous discussion: capi-workgroup/problems#42 & capi-workgroup/api-evolution#22

It seems the consensus of the people involved is that Python.h should be usable with:

• C99 or newer
• C++11 or newer

Now that there's a report about the 3.13 headers containing a C11 feature (anonymous structs/unions), I guess it's time to make an official guideline.