> Adding new C99 features needs a change in PEP 7 (https://www.python.org/dev/peps/pep-0007/#c-dialect)

IMO this PEP is outdated for a long time.

C99 standard is wide. Do we have to explicitly list every single function, macro or constant used by Python? It doesn't sound reasonable to me. IMO saying that we use "C99 except of these few features: <...>" would be closer to the reality. I don't know which features are not used.

Well, if you ask me, I would simply require a C99 compiler. That's all :-)

Note: Python uses C11 <stdatomic.h>, but it remains an optional requirement.