[licenses] Update Python files license

Changelog: None
Signed-off-by: Lluis Campos <lluis.campos@northern.tech>

tests/conftest.py

@@ -1,3 +1,17 @@
+# Copyright 2021 Northern.tech AS
+#
+#    Licensed under the Apache License, Version 2.0 (the "License");
+#    you may not use this file except in compliance with the License.
+#    You may obtain a copy of the License at
+#
+#        http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS,
+#    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#    See the License for the specific language governing permissions and
+#    limitations under the License.
+
 import os
 import pytest
 import sys