Fix import

Author: zoidyzoidzoid

tests/unit/trace/ext/django/test_db.py …jango/tests/test_django_db_middleware.pytests/unit/trace/ext/django/test_db.py renamed to contrib/opencensus-ext-django/tests/test_django_db_middleware.py tests/unit/trace/ext/django/test_db.py renamed to contrib/opencensus-ext-django/tests/test_django_db_middleware.py

@@ -40,7 +40,7 @@ def test_process_request(self):
         if django.VERSION < (2, 0):
             pytest.skip("Wrong version of Django")
 
-        from opencensus.trace.ext.django import middleware
+        from opencensus.ext.django import middleware
 
         sql = "SELECT * FROM users"