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/webhooks/github_legacy/view.py

@@ -267,7 +267,7 @@ def api_github_landing(request: HttpRequest, user_profile: UserProfile, event: s
     if event == 'push':
         # If we are given a whitelist of branches, then we silently ignore
         # any push notification on a branch that is not in our whitelist.
-        if branches and short_ref not in re.split('[\s,;|]+', branches):
+        if branches and short_ref not in re.split(r'[\s,;|]+', branches):
             return json_success()
 
     # Map payload to the handler with the right version