Set type of keyword arguments to Dict[unicode, Any] in Python 2.

[rowillia]

This fixes #1954

[gvanrossum]

I've got an inkling there may be other places where the same thing is needed, we just haven't found them yet. If you put the logic in unicode_type() to return str_type() if version >= 3, then you can use just unicode_type() here and we can use it in future places too.

[rowillia]

Done

[gvanrossum]

Sadly I am going to have to revert this. We have a bunch of code that is broken by it. Example:

from typing import Dict

def f(**kw):
    # type: (**int) -> None
    a = {}  # type: Dict[str, int]
    a = kw

The error is

__tmp__.py:6: error: Incompatible types in assignment (expression has type Dict[unicode, int], variable has type Dict[str, int])

(It's easy to construct other examples too, using kw[...] or kw.keys().)