Add Apache 2.0 license to source file

As per OpenStack licensing guide lines [1]:
[H102 H103] Newly contributed Source Code should be licensed under
the Apache 2.0 license.
[H104] Files with no code shouldn't contain any license header nor
comments, and must be left completely empty.

[1] http://docs.openstack.org/developer/hacking/#openstack-licensing

Change-Id: I9f4673f19ce045a2aad656fdc27679a8ada23ff7

tools/list_weeks.py

@@ -1,5 +1,17 @@
 #!/usr/bin/env python
 
+#  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.
+
 from __future__ import print_function
 
 import argparse