Raise MiddlewareNotUsed in deprecated FlagConditionsMiddleware

Per the [Django documentation](https://docs.djangoproject.com/en/2.2/topics/http/middleware/#marking-middleware-as-unused) on marking middle as unused, this change raises `MiddlewareNotUsed` when initializing the deprecated `FlagConditionsMiddleware`. This way, Django takes the middleware out of the middleware process and the fact that this middleware no longer calls `get_response` and returns it for a request will not interrupt the flow of the request through other middleware.

flags/middleware.py

@@ -1,5 +1,7 @@
 import warnings
 
+from django.core.exceptions import MiddlewareNotUsed
+
 
 class FlagConditionsMiddleware:
     def __init__(self, get_response):
@@ -8,6 +10,7 @@ def __init__(self, get_response):
             "effect. It will be removed in a future version of Django-Flags. ",
             FutureWarning,
         )
+        raise MiddlewareNotUsed
 
     def __call__(self, request):
-        pass
+        pass  # pragma: no cover

flags/tests/test_middleware.py

@@ -1,5 +1,6 @@
 import warnings
 
+from django.core.exceptions import MiddlewareNotUsed
 from django.test import SimpleTestCase
 
 from flags.middleware import FlagConditionsMiddleware
@@ -8,7 +9,8 @@
 class FlagConditionsMiddlewareTests(SimpleTestCase):
     def test_middleware_raises_warning(self):
         with warnings.catch_warnings(record=True) as warning_list:
-            FlagConditionsMiddleware(None)
+            with self.assertRaises(MiddlewareNotUsed):
+                FlagConditionsMiddleware(None)
 
             self.assertEqual(len(warning_list), 1)
             self.assertEqual(warning_list[0].category, FutureWarning)