Correctly handle async inner functions

[adamrothman]

This appears to be similar to #1551 but is somewhat more of a problem as the error occurs even when mypy attempts to parse un-annotated code.

mypy doesn't mind this code at all:

def get_printer(thing):
    def printer():
        print(thing)

    return printer

But if the inner function is defined as a coroutine using async/await syntax, like so:

def get_async_printer(thing):
    async def aprinter():
        print(thing)

    return aprinter

mypy throws a bunch of errors:

$ mypy ./mypy-workout.py
mypy-workout.py: note: In function "get_async_printer":
mypy-workout.py:9: error: Parse error before "def"
mypy-workout.py:9: error: Parse error before :
mypy-workout.py:10: error: Inconsistent indentation
mypy-workout.py: note: At top level:
mypy-workout.py:15: error: Inconsistent indentation

This happens whether the file is annotated with type hints or not.

[gvanrossum]

Support for PEP 492 syntax is entirely absent. We plan to add it after the fast parser.

…

--Guido (mobile)

[adamrothman]

Oops! Don't know how I missed #1353... Well, thanks for responding so quickly. I'll follow that issue.

[gvanrossum]

Dupe of #1453.