Fix "invalid escape sequence" when running with -Werror

```
$ python3.7 -Werror
Python 3.7.0rc1 (default, Jun 16 2018, 03:32:08) 
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> '# noqa(?:: (?P<codes>([A-Z][0-9]+(?:[,\s]+)?)+))?'
  File "<stdin>", line 1
SyntaxError: invalid escape sequence \s
```

src/flake8/defaults.py

@@ -46,7 +46,7 @@
     # We do not care about the ``: `` that follows ``noqa``
     # We do not care about the casing of ``noqa``
     # We want a comma-separated list of errors
-    '# noqa(?:: (?P<codes>([A-Z][0-9]+(?:[,\s]+)?)+))?',
+    r'# noqa(?:: (?P<codes>([A-Z][0-9]+(?:[,\s]+)?)+))?',
     re.IGNORECASE
 )