kerberos: fix sorting Principal objects

When service-find was issued under Python 3, the command fails
because it tried to sort a list of Principal objects which was not
possible.

https://pagure.io/freeipa/issue/4985

Reviewed-By: Felipe Volpone <fbarreto@redhat.com>

ipapython/kerberos.py

@@ -93,6 +93,18 @@ class Principal(object):
     def __ne__(self, other):
         return not self.__eq__(other)
 
+    def __lt__(self, other):
+        return unicode(self) < unicode(other)
+
+    def __le__(self, other):
+        return self.__lt__(other) or self.__eq__(other)
+
+    def __gt__(self, other):
+        return not self.__le__(other)
+
+    def __ge__(self, other):
+        return self.__gt__(other) or self.__eq__(other)
+
     def __hash__(self):
         return hash(self.components + (self.realm,))