Preserve synchronous iteration over awaited collections

[EiNSTeiN-]

I am opening this PR to preserve synchronous iteration over an awaited collection. The code below currently decompiles with the wrong iteration protocol:

async def select(read):
    result = []
    for item in await read():
        result.append(item)
    return result

The loop is currently emitted as:

    async for item in read():
        result.append(item)

The producer must be awaited once and its result then consumed as a regular iterable through __iter__. An async for instead requires the asynchronous iteration protocol and no longer performs the source's await read() operation. A producer returning an ordinary list therefore no longer works.

When the FOR_ITER handler sees an Await expression, it currently removes the await wrapper and marks the loop asynchronous. With this change, FOR_ITER always selects synchronous iteration. GET_AITER/GET_ANEXT retain their separate genuine-async path, so async for item in read() keeps its existing meaning.

The source syntax begins in Python 3.5; the repair is not limited to a recent bytecode version.