C-API for initializing statically linked extension modules

[vstinner (Victor Stinner)]

@vstinner

By the way, it would be nice to move “inittab” configuration into a “stateful” API like PyConfig, or currently discussed PEP 741: Python Configuration C API , rather than “stateless” function calls such as `PyImport_

ExtendInittab()`.

I updated PEP 741 to add PyInitConfig_AddModule() function, see: PEP 741: Python Configuration C API (second version) discussion.