docs: def python_2_div_compatible(klass):
      """
      Adds a `__div__()` method to classes that define a `__floordiv__()` method,
      so division works in Python 2. Has no effect in Python 3.
  
      :param klass: The class to modify. Must define `__floordiv__()`.
      :return: The possibly patched class.
      """
      if six.PY2:
          if '__floordiv__' not in klass.__dict__:
              raise ValueError(
                  '@python_2_div_compatible cannot be applied to {0} because it '
                  'doesn\'t define __floordiv__().'.format(klass.__name__))
          klass.__div__ = klass.__floordiv__
      return klass