py/compile: Do not await __aiter__ special method return value.

[jonathanhogg]

As per discussion in #6267, the final PEP492 spec says that the __aiter__() method should not return an awaitable. This PR changes the compiler to not compile in an await and updates the async for tests to match.

Note that the test in async_for2.py has had to be more modified in behaviour as well as syntax as it can no longer yield in the __aiter__() method. I've just removed the call to await f() and the matching output completely.

Have run the UNIX port tests successfully on macOS and also tested with my own async iterable code on the ESP32 port.

Note: This will be a breaking change to any (not precompiled) code that currently defines __aiter__ methods using await.

[peterhinch]

The transition to uasyncio V3 introduces other breaking changes in the interests of CPython compatibility, so in my view now is the time to do this.

[peterhinch]

CPython's syntax rules have changed. V3.6.9 accepts either

    async def __aiter__(self):
        print('aiter')
        return self

or

    def __aiter__(self):
        print('aiter')
        return self

CPython 3.8.0 accepts only the latter, throwing a cryptic exception in the former case.

[jonathanhogg]

CPython's syntax rules have changed. V3.6.9 accepts either
[...]
CPython 3.8.0 accepts only the latter, throwing a cryptic exception in the former case.

I think this is consistent with this message on the original CPython bug, which suggested that the change be staggered in with a PendingDeprecationWarning in 3.5.2, a DeprecationWarning in 3.6 and then dropping support for async __aiter__() in 3.7.

I've just tested on a 3.7 build and using async fails:

  File "aiter_test2.py", line 26, in main
    async for i in SlowCounter(10):
TypeError: 'async for' received an object from __aiter__ that does not implement __anext__: coroutine

[dpgeorge]

The transition to uasyncio V3 introduces other breaking changes in the interests of CPython compatibility, so in my view now is the time to do this.

I was thinking to wait until after v1.13 is released to change/fix this, but, I think you're right, let's do it now.

[dpgeorge]

Rebased, squashed (in this case it's good to have the code change go with the change to the tests) and merged in 37e1b5c

Thank you!