Raise AttributeError if no app provider has it

whoops, super().__getattr__() is not really defined

tests/test_app.py

@@ -3,6 +3,7 @@
 import os
 import shutil
 import tempfile
+import warnings
 from unittest import TestCase
 from unittest.mock import patch, MagicMock
 
@@ -127,7 +128,9 @@ class TestAppProvider(TestCase):
         self.assertIs(provider.app, self.app)
 
         # but can pass app handler instead
-        provider = app.AppProvider(self.app)
+        with warnings.catch_warnings():
+            warnings.filterwarnings('ignore', category=DeprecationWarning)
+            provider = app.AppProvider(self.app)
         self.assertIs(provider.config, self.config)
         self.assertIs(provider.app, self.app)