Django 3.0 compat: Avoid pickle of local object

If the FireSimpleEvent class is defined in the function, then an error
is raised:

AttributeError: Can't pickle local object 'EventUnionTests.test_fire.<locals>.FireSimpleEvent'

Pickling works when the class is defined at the module level, but the
object attribute .called is set on the deserialized copy, not the
original. Refactor the test class to work on a global variable instead.

tests/test_events.py

@@ -42,6 +42,16 @@ class FilteredContentTypeEvent(ContentTypeEvent):
     filters = set(['color', 'flavor'])
 
 
+fire_simple_event_called = False
+
+
+class FireSimpleEvent(SimpleEvent):
+    def _mails(self, users_and_watches):
+        global fire_simple_event_called
+        fire_simple_event_called = True
+        return []
+
+
 class UsersWatchingTests(TestCase):
     """Unit tests for Event._users_watching_by_filter()"""
 
@@ -262,19 +272,13 @@ def _users_watching(self):
 
     def test_fire(self):
         """Assert firing the union gets the mails from the first event."""
-
-        class FireSimpleEvent(SimpleEvent):
-            called = False
-
-            def _mails(self, users_and_watches):
-                self.called = True
-                return []
-
+        global fire_simple_event_called
         watch(event_type=TYPE, email='he@llo.com').save()
         simple_event = FireSimpleEvent()
+        fire_simple_event_called = False
         another_event = AnotherEvent()
         EventUnion(simple_event, another_event).fire()
-        self.assertTrue(simple_event.called)
+        self.assertTrue(fire_simple_event_called)
 
     def test_watch_lists(self):
         """Ensure the Union returns every watch a user has."""