Improve known metaclass base class detection

We previously only looked for known metaclass base classes that were
inheriting using the simple `ast.Name` syntax:

    class Meta(type): pass
    class Meta(ABCMeta): pass

We weren't accounting for the `ast.Attribute` syntax:

    class Meta(abc.ABCMeta): pass

This second syntax will now also match `ABCMeta` and be recognized as a
metaclass base class.

We could alternatively add `abc.ABCMeta` to METACLASS_BASES and
construct the fully-qualified name of each base class that uses the
`ast.Attribute` syntax. The simpler approach used here is currently
unambiguous, so we'll start with this.

src/pep8ext_naming.py

@@ -4,6 +4,7 @@
 from collections import deque
 from fnmatch import fnmatch
 from functools import partial
+from itertools import chain
 
 from flake8_polyfill import options
 
@@ -212,11 +213,15 @@ def tag_class_functions(self, cls_node):
             if isinstance(meth, ast.Name):
                 late_decoration[meth.id] = self.decorator_to_type[func_name]
 
-        cls_bases = [b for b in cls_node.bases if isinstance(b, ast.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_bases if name.id in METACLASS_BASES)
+        # If this class inherits from a known metaclass base class, it is
+        # itself a metaclass, and we'll consider all of it's methods to be
+        # classmethods.
+        bases = chain(
+            (b.id for b in cls_node.bases if isinstance(b, ast.Name)),
+            (b.attr for b in cls_node.bases if isinstance(b, ast.Attribute)),
+        )
+        ismetaclass = any(name for name in bases if name in METACLASS_BASES)
+
         self.set_function_nodes_types(
             iter_child_nodes(cls_node), ismetaclass, late_decoration)
 

testsuite/N805.py

@@ -1,3 +1,4 @@
+import abc
 from abc import ABCMeta
 
 #: Okay
@@ -62,6 +63,12 @@ def __new__(cls, name, bases, attrs):
         pass
     def test(cls):
         pass
+#: Okay
+class Meta(abc.ABCMeta):
+    def __new__(cls, name, bases, attrs):
+        pass
+    def test(cls):
+        pass
 #: Okay(--classmethod-decorators=clazzy,cool)
 class NewClassmethodDecorators(object):
     @clazzy