bpo-45123: PyAiter_Check and PyObject_GetAiter fix & rename. (GH-28194) (GH-28199)

Fix PyAiter_Check to only check for the `__anext__` presense (not for
`__aiter__`). Rename `PyAiter_Check()` to `PyAIter_Check()`,
`PyObject_GetAiter()` -> `PyObject_GetAIter()`.
diff --git a/Doc/c-api/object.rst b/Doc/c-api/object.rst
index 70cff69..17e3707 100644
--- a/Doc/c-api/object.rst
+++ b/Doc/c-api/object.rst
@@ -358,7 +358,7 @@
    iterated.
 
 
-.. c:function:: PyObject* PyObject_GetAiter(PyObject *o)
+.. c:function:: PyObject* PyObject_GetAIter(PyObject *o)
 
    This is the equivalent to the Python expression ``aiter(o)``. Takes an
    :class:`AsyncIterable` object and returns an :class:`AsyncIterator` for it.