ExtensionClasses in isinstance() and issubclass().

  - abstract instance and class protocols are used *only* in those
    cases that would generate errors before the patch.  That is, there's
    no penalty for the normal case.

  - instance protocol: an object smells like an instance if it
    has a __class__ attribute that smells like a class.

  - class protocol: an object smells like a class if it has a
    __bases__ attribute that is a tuple with elements that
    smell like classes (although not all elements may actually get
    sniffed ;).