Add a license header and comment. (#11479)

changelog.d/11479.feature

+Add plugin support for controlling database background updates.

tests/storage/test_background_update.py

+# Copyright 2021 The Matrix.org Foundation C.I.C.
+#
+# 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.
+
+# Use backported mock for AsyncMock support on Python 3.6.
 from mock import Mock
 
 from twisted.internet.defer import Deferred, ensureDeferred