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/push_notifications.py

@@ -444,7 +444,7 @@ def get_mobile_push_content(rendered_content: str) -> str:
         # Convert default emojis to their unicode equivalent.
         classes = elem.get("class", "")
         if "emoji" in classes:
-            match = re.search("emoji-(?P<emoji_code>\S+)", classes)
+            match = re.search(r"emoji-(?P<emoji_code>\S+)", classes)
             if match:
                 emoji_code = match.group('emoji_code')
                 char_repr = ""