Add missing license header.

PiperOrigin-RevId: 605344949
Change-Id: Ib65b44b02f86f14a75d350ed79fdac596ee63022

open_spiel/python/voting/examples/chatbot_arena.py

@@ -1,3 +1,17 @@
+# Copyright 2023 DeepMind Technologies Limited
+#
+# 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.
+
 """Chat bot Arena dataset."""
 
 # pylint: disable=unused-import