Python密码学:无法使用PKCS1v15填充使用RSA私钥签名

我试图使用SHA1哈希实现Python和密码学库对PHP的openssl_pkey_get_private和openssl_sign的功能等效签名。我读到PHP使用PKCS1v15填充，所以这就是我试图使用的。我的代码是:

from cryptography.hazmat.primitives import hashes
from cryptography.hazmat.primitives.asymmetric import padding
from cryptography.hazmat.primitives.serialization import load_pem_private_key
from cryptography.hazmat.backends import default_backend
pk = open('key.pem', 'rb')
key = load_pem_private_key(pk.read(), password=None, backend=default_backend())
message = b'hello world'
signature = key.sign(
    message,
    padding.PKCS1v15,
    hashes.SHA1()
)

执行结果为:

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-21-ef3db8a6f4a8> in <module>()
      3     message,
      4     padding.PKCS1v15,
----> 5     hashes.SHA1()
      6 )
/home/vagrant/virtualenvs/test/lib/python3.5/site-packages/cryptography/hazmat/backends/openssl/rsa.py in sign(self, data, padding, algorithm)
    613 
    614     def sign(self, data, padding, algorithm):
--> 615         signer = self.signer(padding, algorithm)
    616         signer.update(data)
    617         signature = signer.finalize()
/home/vagrant/virtualenvs/test/lib/python3.5/site-packages/cryptography/hazmat/backends/openssl/rsa.py in signer(self, padding, algorithm)
    550 
    551     def signer(self, padding, algorithm):
--> 552         return _RSASignatureContext(self._backend, self, padding, algorithm)
    553 
    554     def decrypt(self, ciphertext, padding):
/home/vagrant/virtualenvs/test/lib/python3.5/site-packages/cryptography/hazmat/backends/openssl/rsa.py in __init__(self, backend, private_key, padding, algorithm)
    170 
    171         if not isinstance(padding, AsymmetricPadding):
--> 172             raise TypeError("Expected provider of AsymmetricPadding.")
    173 
    174         self._pkey_size = self._backend._lib.EVP_PKEY_size(
TypeError: Expected provider of AsymmetricPadding.