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_home.py

@@ -318,7 +318,7 @@ class HomeTest(ZulipTestCase):
     def _get_page_params(self, result: HttpResponse) -> Dict[str, Any]:
         html = result.content.decode('utf-8')
         lines = html.split('\n')
-        page_params_line = [l for l in lines if re.match('^\s*var page_params', l)][0]
+        page_params_line = [l for l in lines if re.match(r'^\s*var page_params', l)][0]
         page_params_json = page_params_line.split(' = ')[1].rstrip(';')
         page_params = ujson.loads(page_params_json)
         return page_params