Use license_files instead of license_file

See: pypa/setuptools#2620

setup.cfg

@@ -1,4 +1,4 @@
-# Copyright (C) 2020, 2021 Serghei Iakovlev <egrep@protonmail.ch>
+# Copyright (C) 2020, 2021, 2022 Serghei Iakovlev <egrep@protonmail.ch>
 #
 # This file is free software; you can redistribute it and/or
 # modify it under the terms of the GNU General Public License
@@ -14,7 +14,7 @@
 # along with this file.  If not, see <https://www.gnu.org/licenses/>.
 
 [metadata]
-license_file = LICENSE
+license_files = LICENSE
 description_file = README.rst
 long_description_content_type = text/x-rst