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>

tools/lib/find_add_class.py

@@ -81,7 +81,7 @@ def find(fns):
         for i, line in enumerate(lines):
             if 'addClass' in line:
                 html_classes = []  # type: List[str]
-                m = re.search('addClass\([\'"](.*?)[\'"]', line)
+                m = re.search(r'''addClass\(['"](.*?)['"]''', line)
                 if m:
                     html_classes = [m.group(1)]
                 if not html_classes: