Fix false E251 error involving square brackets

Fixed a case where a comma is erroneously interpreted as an
argument separator when square brackets are used in an argument
annotation with a default value.

Closes #496

Author: agronholm

pep8.py

@@ -794,9 +794,9 @@ def whitespace_around_named_parameter_equals(logical_line, tokens):
             if start != prev_end:
                 yield (prev_end, message)
         if token_type == tokenize.OP:
-            if text == '(':
+            if text in '([':
                 parens += 1
-            elif text == ')':
+            elif text in ')]':
                 parens -= 1
             elif in_def and text == ':' and parens == 1:
                 annotated_func_arg = True

testsuite/E25.py

@@ -32,5 +32,6 @@ def foo(bar = False):
 
 # Annotated Function Definitions
 #: Okay
-def munge(input: AnyStr, sep: AnyStr = None, limit=1000) -> AnyStr:
+def munge(input: AnyStr, sep: AnyStr = None, limit=1000,
+          extra: Union[str, dict] = None) -> AnyStr:
     pass