Type annotations on `with` statements

[rwbarton]

We might not always be able to infer the type of the variable bound by a with statement, just like we might not always be able to infer the type of a variable being assigned to for the first time. For this minimal example

from typing import TypeVar
from contextlib import ContextManager
T = TypeVar('T')
def g() -> ContextManager[T]: ...

with g() as x:
    pass

mypy reports

with.py:6: error: Need type annotation for variable

It would be nice to be able to write, for example,

with g() as x:  # type: int

declaring the type of x. This is currently rejected by mypy as a parse error.

[gvanrossum]

Yeah, PEP 484 says this should work. (Also for a for loop.)

[gvanrossum]

Actually we already have an issue for this. Closing as dupe of #892.