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 +`.

Xlib/xauth.py

@@ -97,7 +97,7 @@ def __getitem__(self, i):
         return self.entries[i]
 
     def get_best_auth(self, family, address, dispno,
-                      types = ( "MIT-MAGIC-COOKIE-1", )):
+                      types = ( b"MIT-MAGIC-COOKIE-1", )):
 
         """Find an authentication entry matching FAMILY, ADDRESS and
         DISPNO.
@@ -110,7 +110,8 @@ def get_best_auth(self, family, address, dispno,
         otherwise XNoAuthError is raised.
         """
 
-        num = str(dispno)
+        num = str(dispno).encode()
+        address = address.encode()
 
         matches = {}