Adds a `__nonzero__()` method to classes that define a `__bool__()` method,
    so boolean conversion works in Python 2. Has no effect in Python 3.

    :param klass: The class to modify. Must define `__bool__()`.
    :return: The possibly patched class.

def python_2_nonzero_compatible(klass):
    """
    Adds a `__nonzero__()` method to classes that define a `__bool__()` method,
    so boolean conversion works in Python 2. Has no effect in Python 3.

    :param klass: The class to modify. Must define `__bool__()`.
    :return: The possibly patched class.
    """
    if six.PY2:
        if '__bool__' not in klass.__dict__:
            raise ValueError(
                '@python_2_nonzero_compatible cannot be applied to {0} because '
                'it doesn\'t define __bool__().'.format(klass.__name__))
        klass.__nonzero__ = klass.__bool__
    return klass