Enable pycodestyle W605 (invalid escape sequence).

The only changes visible at the AST level, checked using
https://github.com/asottile/astpretty, are

zerver/lib/test_fixtures.py:
'\x1b\\[(1|0)m' ↦ '\\x1b\\[(1|0)m'
'\\[[X| ]\\] (\\d+_.+)\n' ↦ '\\[[X| ]\\] (\\d+_.+)\\n'

which is fine because re treats '\\x1b' and '\\n' the same way as
'\x1b' and '\n'.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

zerver/tests/test_middleware.py

@@ -38,7 +38,7 @@ class SlowQueryTest(ZulipTestCase):
         self.assertIn("logs", str(last_message.recipient))
         self.assertEqual(last_message.topic_name(), "testserver: slow queries")
         self.assertRegexpMatches(last_message.content,
-                                 "123\.456\.789\.012 SOCKET  200 10\.\ds .*")
+                                 r"123\.456\.789\.012 SOCKET  200 10\.\ds .*")
 
     @override_settings(ERROR_BOT=None)
     @patch('logging.info')