It is now considered a historical accident that e.g. `for` loops and the `iter()` built-in function do not require the iterators they work with to define `__iter__`, only `__next__`.
(cherry picked from commit be36e06340)
Co-authored-by: Brett Cannon <brett@python.org>
Fix PyAiter_Check to only check for the `__anext__` presense (not for
`__aiter__`). Rename `PyAiter_Check()` to `PyAIter_Check()`,
`PyObject_GetAiter()` -> `PyObject_GetAIter()`.
I think that none of these API calls can fail, but only few of them are
documented as such. Add the sentence "This function always succeeds" (which is
the same already used e.g. by PyNumber_Check) to all of them.
The added parentheses around the PyIter_Next assignment suppress the following warning which gcc throws without:
```
warning: using the result of an assignment as a condition without parentheses [-Wparentheses]
```
The other change is a typo fix
Jelle Zijlstra