title: Callable is invalid base class?
body: <p>Can someone explain why inheriting from unparameterized and parameterized <code>Callable</code>:</p>
  
  <pre><code>from typing import Callable
  from typing import NoReturn
  from typing import TypeVar
  
  
  T = TypeVar('T', str, int)
  C = Callable[[T], NoReturn]
  
  
  class Foo(Callable):
  
      def __call__(self, t: T):
          pass
  
  
  class Bar(C):
  
      def __call__(self, t: T):
          pass
  </code></pre>
  
  <p>when passed to mypy raises errors for both <code>Foo</code> and <code>Bar</code>:</p>
  
  <pre><code>tmp.py:13: error: Invalid base class
  tmp.py:19: error: Invalid base class
  </code></pre>