Use license_files instead of license_file

Closes borgbackup#6727

See: pypa/setuptools#2620

setup.cfg

@@ -25,7 +25,7 @@ classifiers =
     Topic :: System :: Archiving :: Backup
 platforms = Linux, MacOS X, FreeBSD, OpenBSD, NetBSD
 license = BSD
-license_file = LICENSE
+license_files = LICENSE
 project_urls =
     Bug Tracker = https://github.com/borgbackup/borg/issues
     Documentation = https://borgbackup.readthedocs.io