gh-76785: Move _Py_excinfo Functions Out of the Internal C-API (gh-111715)

I added _Py_excinfo to the internal API (and added its functions in Python/errors.c) in gh-111530 (9322ce9).  Since then I've had a nagging sense that I should have added the type and functions in its own PR.  While I do plan on using _Py_excinfo outside crossinterp.c very soon (see gh-111572/gh-111573), I'd still feel more comfortable if the _Py_excinfo stuff went in as its own PR.  Hence, here we are.

(FWIW, I may combine that with gh-111572, which I may, in turn, combine with gh-111573.  We'll see.)