Fix pycodestyle complaints

Apparently, pycodestyle updated itself to now check for invalid escape
sequences, which only complain if the regex in use isn't a raw string
(r"").

src/documents/tests/test_matchables.py

@@ -166,7 +166,7 @@ class TestMatching(TestCase):
     def test_match_regex(self):
 
         self._test_matching(
-            "alpha\w+gamma",
+            r"alpha\w+gamma",
             "MATCH_REGEX",
             (
                 "I have alpha_and_gamma in me",