dirty hack, only import cythonize at the time you use it.

    if you don't write Cython extension,
    you won't fail even if you don't install Cython.

def cythonize(*args, **kwargs):
    '''
    dirty hack, only import cythonize at the time you use it.

    if you don't write Cython extension,
    you won't fail even if you don't install Cython.
    '''
    global cythonize
    from Cython.Build import cythonize
    return cythonize(*args, **kwargs)