title: Pylint cannot handle abstract subclasses of abstract base classes
body: <p>I have a concrete class <code>MyConcreteClass</code> which subclasses an <em>abstract</em>
  class <code>MyABC2</code>, which, in turn, subclasses another <em>abstract</em> class <code>MyABC1</code>:</p>
  
  <pre><code>import abc
  
  class MyABC1 (object):
      __metaclass__ = abc.ABCMeta
      @abc.abstractmethod
      def my_abstract_method (self):
          raise NotImplementedError("MyABC1.my_abstract_method")
  
  class MyABC2 (MyABC1):
      __metaclass__ = abc.ABCMeta
  
  class MyConcreteClass (MyABC2):
      def my_abstract_method (self):
          print "MyConcreteClass.my_abstract_method"
  </code></pre>
  
  <p><code>pylint</code> does not like that I do not define <code>my_abstract_method</code> in <code>MyABC2</code>:</p>
  
  <pre><code>/usr/local/bin/epylint abctest.py
  ************* Module abctest
   abctest.py:9: warning (W0223, abstract-method, MyABC2) Method 'my_abstract_method' is abstract in class 'MyABC1' but is not overridden
  </code></pre>
  
  <p>Is this a known bug?
  Maybe I am doing something wrong?</p>