Python 3.6 invalid escape sequence deprecation fix

https://docs.python.org/3/whatsnew/3.6.html#deprecated-python-behavior

"backslash-character pair that is not a valid escape sequence now
generates a DeprecationWarning. Although this will eventually become a
SyntaxError, that will not be for several Python releases."

Author: scop

pycodestyle.py

@@ -124,7 +124,7 @@
 STARTSWITH_DEF_REGEX = re.compile(r'^(async\s+def|def)')
 STARTSWITH_TOP_LEVEL_REGEX = re.compile(r'^(async\s+def\s+|def\s+|class\s+|@)')
 STARTSWITH_INDENT_STATEMENT_REGEX = re.compile(
-    r'^\s*({0})'.format('|'.join(s.replace(' ', '\s+') for s in (
+    r'^\s*({0})'.format('|'.join(s.replace(' ', r'\s+') for s in (
         'def', 'async def',
         'for', 'async for',
         'if', 'elif', 'else',