Fix flake8: remove unused imports in test files

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: nsa-softpay

tests/unit/test_client.py

@@ -1,4 +1,3 @@
-import json
 import unittest
 
 import requests_mock

tests/unit/test_models.py

@@ -7,7 +7,6 @@
 from alertaclient.models.group import Group
 from alertaclient.models.heartbeat import Heartbeat
 from alertaclient.models.key import ApiKey
-from alertaclient.models.note import Note
 from alertaclient.models.permission import Permission
 from alertaclient.models.user import User