CIP8 signing with extended signing key fails

[theeldermillenial]

Describe the bug
When using an extended signing key, either payment or staking, signing a message using cip8.sign produces an error.

To Reproduce
Modify the test_cipy8.py tests to use extended signing keys from the test_keys.py file. Run the test.

Logs
The error generated reads as follows:

pycardano/cip/cip8.py:88: in sign
    encoded = msg.encode()
.venv/lib/python3.10/site-packages/cose/messages/sign1message.py:67: in encode
    message = [self.phdr_encoded, self.uhdr_encoded, self.payload, self.compute_signature()]
.venv/lib/python3.10/site-packages/cose/messages/signcommon.py:63: in compute_signature
    return alg.sign(key=self.key, data=self._sig_structure)
.venv/lib/python3.10/site-packages/cose/algorithms.py:1021: in sign
    sk = Ed25519PrivateKey.from_private_bytes(key.d)
.venv/lib/python3.10/site-packages/cryptography/hazmat/primitives/asymmetric/ed25519.py:77: in from_private_bytes
    return backend.ed25519_load_private_bytes(data)

Expected behavior
Since cip8.sign checks to see if one of the keys is a StakeExtendedSignKey, I would expect that at least a StakeExtendedSignKey to work correctly.

[theeldermillenial]

On a separate note, it might not be a bad idea to parameterize these tests to make it easier.

I'd be happy to take a stab at parameterizing the tests and getting extended signing keys to work.