how to make a PKCS8 RSA signature in python

>>> from Crypto.Signature import PKCS1_v1_5
>>> from Crypto.Hash import SHA
>>> from Crypto.PublicKey import RSA
>>>
>>> message = 'To be signed'
>>> key = RSA.importKey(open('privkey.der').read())
>>> h = SHA.new(message)
>>> signer = PKCS1_v1_5.new(key)
>>> signature = signer.sign(h)