Fix handling of E306 for nested definitions

This was another case of a sloppy regular expression that wasn't made to
catch only *keywords* in the language.

Closes PyCQAgh-619

pycodestyle.py

@@ -122,7 +122,9 @@
 LAMBDA_REGEX = re.compile(r'\blambda\b')
 HUNK_REGEX = re.compile(r'^@@ -\d+(?:,\d+)? \+(\d+)(?:,(\d+))? @@.*$')
 STARTSWITH_DEF_REGEX = re.compile(r'^(async\s+def|def)')
-STARTSWITH_TOP_LEVEL_REGEX = re.compile(r'^(async\s+def|def|class|@)')
+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 (
         'def', 'async def',

testsuite/E30not.py

@@ -155,3 +155,7 @@ class Bar(object):
 classification_errors = None
 #: Okay
 defined_properly = True
+#: Okay
+def foo(x):
+    classification = x
+    definitely = not classification