tests: fix pytest warnings

Fix warnings reported when running unit tests via pytest.

- Rename TestDevice to MockDevice, so that it is not interpreted as a
  test case.
- Fix collections abstract base class imports.
- Add an ini file to ignore the same from "past".

wa/utils/serializer.py

@@ -59,7 +59,8 @@ to specify it explicitly.
 import os
 import re
 import json as _json
-from collections import OrderedDict, Hashable
+from collections import OrderedDict
+from collections.abc import Hashable
 from datetime import datetime
 import dateutil.parser
 import yaml as _yaml  # pylint: disable=wrong-import-order