Consider all metaclass methods to be class methods

It's a widely used (but perhaps not official?) convention to consider a
metaclass's methods to be class methods and to use `cls` as the first
argument (rather than `self`).

Use the simple heuristic of "class inherits from `type`" to identify
metaclasses and mark all of their methods as CLASSMETHOD.

pep8ext_naming.py

@@ -180,13 +180,16 @@ class NamingChecker(object):
             if isinstance(meth, ast.Name):
                 late_decoration[meth.id] = self.decorator_to_type[func_name]
+        # If this class inherits from `type`, it's a metaclass, and we'll
+        # consider all of it's methods to be classmethods.
+        ismetaclass = any(name for name in cls_node.bases if name.id == 'type')
+
         # iterate over all functions and tag them
         for node in iter_child_nodes(cls_node):
             if not isinstance(node, ast.FunctionDef):
                 continue
-
             node.function_type = _FunctionType.METHOD
-            if node.name in ('__new__', '__init_subclass__'):
+            if node.name in ('__new__', '__init_subclass__') or ismetaclass:
                 node.function_type = _FunctionType.CLASSMETHOD
             if node.name in late_decoration:
                 node.function_type = late_decoration[node.name]

testsuite/N804.py

@@ -19,3 +19,11 @@ class NewClassIsRequired(object):
     @cool
     def test(self, sy):
         pass
+#: N804
+class Meta(type):
+    def __new__(self, name, bases, attrs):
+        pass
+#: Okay
+class MetaMethod(type):
+    def test(cls):
+        pass

testsuite/N805.py

@@ -30,6 +30,12 @@ class Foo(object):
 class Foo(object):
     def __init_subclass__(cls):
         pass
+#: Okay
+class Meta(type):
+    def __new__(cls, name, bases, attrs):
+        pass
+    def test(cls):
+        pass
 #: Okay(--classmethod-decorators=clazzy,cool)
 class NewClassmethodDecorators(object):
     @clazzy