rollback action version of cache & add license

Author: mspruc

.github/workflows/backend.yml

@@ -56,7 +56,7 @@ jobs:
         run: |
           python -m pip install --upgrade pip
           pip install -r python/src/pywy/requirements.txt
-      - uses: actions/cache@v5
+      - uses: actions/cache@v4
         with:
           path: ~/.m2/repository
           key: ${{ runner.os }}-maven

python/src/pywy/requirements.txt

@@ -1,3 +1,20 @@
+#
+#  Licensed to the Apache Software Foundation (ASF) under one or more
+#  contributor license agreements.  See the NOTICE file distributed with
+#  this work for additional information regarding copyright ownership.
+#  The ASF licenses this file to You 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.
+#
+
 # we support python 3.11
 numpy==2.3.0
 cloudpickle==3.0.0