Py_FrozenMain was added to the Limited C API in bpo-42591 (3.10.0a4);
but to fix that issue it would be enough to add it to the regular C API.

The function is undocumented, tests were added very recently (bpo-44131),
and most importantly, it is not present in all builds of Python, as
the linker sometimes omits it as unused.
It should be added back when these issues are fixed.

Note that this does not affect Python's regular C API.

https://bugs.python.org/issue43795

Automerge-Triggered-By: GH:encukou