Convert `__anext__` calls on Python 3.10+

Python 3.10 introduces the [`anext`](https://docs.python.org/3/library/functions.html#anext) builtin which calls `__anext__` on the provided async iterator. It would be great if `pyupgrade` could automatically convert legacy `async_iterator.__anext__()` calls into `anext(async_iterator)` upon upgrading to Python 3.10 or above.