bpo-46850: Move _PyInterpreterState_SetEvalFrameFunc() to internal C API (GH-32054)

bpo-46850: Move _PyInterpreterState_SetEvalFrameFunc() to internal C API (GH-32054)

Move the private _PyFrameEvalFunction type, and private
_PyInterpreterState_GetEvalFrameFunc() and
_PyInterpreterState_SetEvalFrameFunc() functions to the internal C
API. The _PyFrameEvalFunction callback function type now uses the
_PyInterpreterFrame type which is part of the internal C API.

Update the _PyFrameEvalFunction documentation.