Finalize support for Python 3.7

Python 3.7 added a warning for a future feature of nested regular expressions. To avoid
this warning we escape what is not a nested regex. This also keeps track of the `async`
keyword and handles it appropriately.

Closes gh-728 

pycodestyle.py

@@ -102,7 +102,7 @@ REPORT_FORMAT = {
 PyCF_ONLY_AST = 1024
 SINGLETONS = frozenset(['False', 'None', 'True'])
-KEYWORDS = frozenset(keyword.kwlist + ['print']) - SINGLETONS
+KEYWORDS = frozenset(keyword.kwlist + ['print', 'async']) - SINGLETONS
 UNARY_OPERATORS = frozenset(['>>', '**', '*', '+', '-'])
 ARITHMETIC_OP = frozenset(['**', '*', '/', '//', '+', '-'])
 WS_OPTIONAL_OPERATORS = ARITHMETIC_OP.union(['^', '&', '|', '<<', '>>', '%'])
@@ -121,7 +121,7 @@ RAISE_COMMA_REGEX = re.compile(r'raise\s+\w+\s*,')
 RERAISE_COMMA_REGEX = re.compile(r'raise\s+\w+\s*,.*,\s*\w+\s*$')
 ERRORCODE_REGEX = re.compile(r'\b[A-Z]\d{3}\b')
 DOCSTRING_REGEX = re.compile(r'u?r?["\']')
-EXTRANEOUS_WHITESPACE_REGEX = re.compile(r'[[({] | []}),;:]')
+EXTRANEOUS_WHITESPACE_REGEX = re.compile(r'[\[({] | [\]}),;:]')
 WHITESPACE_AFTER_COMMA_REGEX = re.compile(r'[,;:]\s*(?:  |\t)')
 COMPARE_SINGLETON_REGEX = re.compile(r'(\bNone|\bFalse|\bTrue)?\s*([=!]=)'
                                      r'\s*(?(1)|(None|False|True))\b')

testsuite/E25.py

@@ -39,7 +39,7 @@ def munge(input: AnyStr, sep: AnyStr = None, limit=1000,
 async def add(a: int = 0, b: int = 0) -> int:
     return a + b
 # Previously E251 four times
-#: E272:1:6
+#: E271:1:6
 async  def add(a: int = 0, b: int = 0) -> int:
     return a + b
 #: E252:1:15 E252:1:16 E252:1:27 E252:1:36

testsuite/E30.py

@@ -157,7 +157,7 @@ def main():
 if __name__ == '__main__':
     main()
 # Previously just E272:1:6 E272:4:6
-#: E302:4:1 E272:1:6 E272:4:6
+#: E302:4:1 E271:1:6 E271:4:6
 async  def x():
     pass

testsuite/E70.py

@@ -14,7 +14,7 @@ del a[:]; a.append(42);
 def f(x): return 2
 #: E704:1:1
 async def f(x): return 2
-#: E704:1:1 E272:1:6
+#: E704:1:1 E271:1:6
 async  def f(x): return 2
 #: E704:1:1 E226:1:19
 def f(x): return 2*x