Refs #27025 -- Fixed "invalid escape sequence" warnings in Python 3.6.

http://bugs.python.org/issue27364

tests/migrations/test_loader.py

@@ -427,7 +427,7 @@ class LoaderTests(TestCase):
     ]})
     def test_loading_squashed_ref_squashed(self):
         "Tests loading a squashed migration with a new migration referencing it"
-        """
+        r"""
         The sample migrations are structured like this:
 
         app_1       1 --> 2 ---------------------*--> 3        *--> 4