auth.TwitterMixin is broken in python 3

[pfrantz]

The TwitterMixin is broken in python 3. When the response is returned an access_token dictionary is created with binary keys and values. This seems to be fine until the _oauth_get_user is called as it assumes the dictionary keys are strings and return value is also a string.

I have patched the routine as follows, but not sure if there are other problems lurking around due to access tokens being bytes.

def _oauth_get_user(self, access_token, callback):
    callback = self.async_callback(self._parse_user_response, callback)
    self.twitter_request(
        "/users/show/" + escape.to_basestring(access_token[b("screen_name")]),
        access_token=access_token, callback=callback)

don't know the code well enough to know whether its better to have access_tokens all converted to strings or leave them in byte format.

[bdarnell]

This is fixed in the master branch with e7485f8

I've added some more tests so this should be enough, but please let me know if you see any other issues.