Ensures :attr:`subject` is an int, float, or long.

def is_numeric(self):
        """
        Ensures :attr:`subject` is an int, float, or long.
        """
        from decimal import Decimal
        numeric_types = (int, float, long, Decimal) if USING_PYTHON2 else (int, float, Decimal)  # noqa
        if not isinstance(self._subject, numeric_types):
            raise self._error_factory(_format("Expected {} to be numeric (int, float, long or Decimal)", self._subject))