Fix support for async for comprehensions

[JelleZijlstra]

Fixes #2924

[gvanrossum]

Thanks!

[gvanrossum]

So I have some bad news.

Running mypy's own tests with these I see two new errors, one in crawl.py and on in crawl2.py. The offending line is

ipaddrs = yield from self.loop.getaddrinfo(host, port)

In the latter it uses await instead.

It's easy to repro manually:

$ mypy test-data/samples/crawl.py
test-data/samples/crawl.py:147: error: 'Union[Tuple[builtins.str, builtins.int], Tuple[builtins.str, builtins.int, builtins.int, builtins.int]]' object is not iterable

[JelleZijlstra]

Oops, I'll look into that tonight.

[JelleZijlstra]

Are you sure the errors are due to this change? I can't currently reproduce it (maybe I'm installing mypy master wrong), but I suspect it's due to python/typeshed#958, which among other things made the return type of loop.getaddrinfo more precise. It was just tuple, but I changed it to a union of the two kinds of tuple it can actually return (for IPv4 and IPv6 addresses I think).

[gvanrossum]

Yes, sorry. I was confused about which tracker/PR to blame it to.