Return None instead of raising an exception on missing, valid emeter keys (#146)

Fixes #142

Also, update the pre-commit hooks to their newest versions

kasa/tests/test_readme_examples.py

@@ -1,8 +1,8 @@
 import sys
 
 import pytest
-
 import xdoctest
+
 from kasa.tests.conftest import get_device_for_file