'Construct a Signer instance from a string.
Args:
key: string, private key in P12 format.
password: string, password for the private key file.
Returns:
Signer instance.
Raises:
OpenSSL.crypto.Error if the key can\'t be parsed.'

@staticmethod
def from_string(key, password='notasecret'):

    pkey = crypto.load_pkcs12(key, password).get_privatekey()
    return Signer(pkey)
