Fix get_best_auth for Python 3

All self.entries items are byte strings, but get_best_auth receives
Unicode strings. Convert them to bytes before comparing.

This fixes a bug that prevented python3-xlib to find X11 cookies in
Xauthority, thus not allowing to use it without `xhost +`.
Merge pull request #7 from ntrrgc/master

Fix get_best_auth for Python 3