Types from the typing module that don't exist in the specified Python version are not detected by Mypy

Hello there,

I think this is not the desired behaviour of Mypy. If I set the version of Python to 3.5, it doesn't report Deque imports, even though this type only appeared in Python 3.6.

mypy.ini:
```
[mypy]
python_version = 3.5
```

The python code:
```
from typing import Deque

...
```

This does not report any errors with mypy 0.670.