Update copyright for 2021

We are in 2021. Since 6 months.

apps/activity/api/serializers.py

-# Copyright (C) 2018-2020 by BDE ENS Paris-Saclay
+# Copyright (C) 2018-2021 by BDE ENS Paris-Saclay
 # SPDX-License-Identifier: GPL-3.0-or-later
 
 from rest_framework import serializers