PrivateKey and PublicKey objects missing sign/verify with NetHSM PKCS#11 backend

When using python-pkcs11 with a NetHSM PKCS#11 backend, PrivateKey and PublicKey objects retrieved via session.get_key() do not expose sign or verify at runtime, despite SignMixin and VerifyMixin existing in the type hierarchy.

Steps to Reproduce

import pkcs11

lib = pkcs11.lib("/path/to/nethsm-pkcs11.so")
token = lib.get_token(token_label="mytoken")

with token.open(user_pin="mypin") as session:
    private_key = session.get_key(object_class=pkcs11.ObjectClass.PRIVATE_KEY, label="mykey")
    private_key.sign(b"hello", mechanism=pkcs11.Mechanism.SHA256_RSA_PKCS_PSS)

Expected Behavior

PrivateKey objects returned by session.get_key() should support .sign() and PublicKey objects should support .verify(), consistent with how other backends (e.g. SoftHSM2) behave.

Actual Behavior

private_key.sign(digest, mechanism=mechanism, mechanism_param=mechanism_param)
^^^^^^^^^^^^^^^^
AttributeError: 'PrivateKey' object has no attribute 'sign'

public_key.verify(digest, signature, mechanism=mechanism, mechanism_param=mechanism_param)
^^^^^^^^^^^^^^^^^
AttributeError: 'PublicKey' object has no attribute 'verify'

Environment

• python-pkcs11: v0.9.4
• OS: Windows 11
• PKCS#11 backend: NetHSM v2.1.0 x86_64

Start at session.get_key() and inspect how retrieved PrivateKey and PublicKey objects relate to the existing SignMixin and VerifyMixin. Reproduce the NetHSM behavior from the issue, then verify that retrieved keys expose sign() and verify() consistently with the expected backend behavior.