'Construct a Signer instance from a string.
Args:
key: string, private key in PKCS12 or PEM 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'):

    key = _helpers._to_bytes(key)
    parsed_pem_key = _helpers._parse_pem_key(key)
    if parsed_pem_key:
        pkey = crypto.load_privatekey(crypto.FILETYPE_PEM, parsed_pem_key)
    else:
        password = _helpers._to_bytes(password, encoding='utf-8')
        pkey = crypto.load_pkcs12(key, password).get_privatekey()
    return OpenSSLSigner(pkey)
