gh-114626: Add again _PyCFunctionFastWithKeywords name (GH-115561)

Keep the old private _PyCFunctionFastWithKeywords name (Python 3.7)
as an alias to the new public name PyCFunctionFastWithKeywords
(Python 3.13a4).

_PyCFunctionWithKeywords doesn't exist in Python 3.13a3, whereas
_PyCFunctionFastWithKeywords was removed in Python 3.13a4.