Bad typing for Python 2.7 with `unicode_literals`

Consider:
```py
from __future__ import unicode_literals
import sys

print(sys.modules['sys'])
```

`mypy --py2 unicode_literals.py` results in:
```
unicode_literals.py:4: error: Invalid index type "unicode" for "Dict[str, Any]"; expected type "str"
```

However, `python2.7 unicode_literals.py` results in:
```
<module 'sys' (built-in)>
```