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/lib/test_classes.py

@@ -353,7 +353,7 @@ class ZulipTestCase(TestCase):
         from django.core.mail import outbox
         if url_pattern is None:
             # This is a bit of a crude heuristic, but good enough for most tests.
-            url_pattern = settings.EXTERNAL_HOST + "(\S+)>"
+            url_pattern = settings.EXTERNAL_HOST + r"(\S+)>"
         for message in reversed(outbox):
             if email_address in message.to:
                 return re.search(url_pattern, message.body).groups()[0]