api: make enforce() static

It doesn't use 'self'. And pylint complains.

qubes/api/__init__.py

@@ -196,7 +196,8 @@ class AbstractQubesAPI(object):
         return apply_filters(iterable,
             self.fire_event_for_permission(**kwargs))
 
-    def enforce(self, predicate):
+    @staticmethod
+    def enforce(predicate):
         '''An assert replacement, but works even with optimisations.'''
         if not predicate:
             raise PermissionDenied()