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/linter_lib/pep8.py

@@ -41,7 +41,7 @@ def check_pep8(files):
         'E226',
 
         # New rules in pycodestyle 2.4.0 that we haven't decided whether to comply with yet
-        'E252', 'W605', 'W504',
+        'E252', 'W504',
 
         # "multiple spaces after ':'"
         # This is the `{}` analogue of E221, and these are similarly being used