How to use the buku.__version__ function in buku

@only_python_3_5
@pytest.mark.parametrize(
    'status_code, latest_release',
    product([200, 404], [True, False])
)
def test_check_upstream_release(status_code, latest_release):
    """test func."""
    resp = mock.Mock()
    resp.status = status_code
    m_manager = mock.Mock()
    m_manager.request.return_value = resp
    with mock.patch('buku.urllib3') as m_urllib3, \
            mock.patch('buku.print') as m_print:
        import buku
        if latest_release:
            latest_version = 'v{}'.format(buku.__version__)
        else:
            latest_version = 'v0'
        m_urllib3.PoolManager.return_value = m_manager
        resp.data.decode.return_value = json.dumps([{'tag_name': latest_version}])
        buku.check_upstream_release()
        if status_code != 200:
            return
        len(m_print.mock_calls) == 1

def get_custom_version(ctx, param, value):
    if not value or ctx.resilient_parsing:
        return
    message = '%(app_name)s %(app_version)s\nFlask %(version)s\nPython %(python_version)s'
    click.echo(message % {
        'app_name': 'Buku',
        'app_version': __version__,
        'version': flask_version,
        'python_version': sys.version,
    }, color=ctx.color)
    ctx.exit()