bpo-45123: PyAiter_Check and PyObject_GetAiter fix & rename. (GH-28194)

Fix PyAiter_Check to only check for the `__anext__` presense (not for
`__aiter__`). Rename `PyAiter_Check()` to `PyAIter_Check()`,
`PyObject_GetAiter()` -> `PyObject_GetAIter()`.

Co-authored-by: Pablo Galindo Salgado <Pablogsal@gmail.com>