Make project REUSE-compliant

Makefile

@@ -1,20 +1,7 @@
-# Night Theme Switcher Gnome Shell extension
-#
-# Copyright (C) 2019, 2020 Romain Vigier
-#
-# This program is free software: you can redistribute it and/or modify it under
-# the terms of the GNU General Public License as published by the Free Software
-# Foundation, either version 3 of the License, or (at your option) any later
-# version.
-#
-# This program is distributed in the hope that it will be useful, but WITHOUT
-# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
-# FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
-#
-# You should have received a copy of the GNU General Public License along with
-# this program. If not, see <http s ://www.gnu.org/licenses/>.
+# SPDX-FileCopyrightText: 2019-2021 Romain Vigier <contact AT romainvigier.fr>
+# SPDX-License-Identifier: GPL-3.0-or-later
 
-COPYRIGHT_YEAR = 2019, 2020
+COPYRIGHT_YEAR = 2019-2021
 AUTHOR_NAME = Romain Vigier
 AUTHOR_EMAIL =