setup.cfg: Replace deprecated license_file with license_files

Replace the deprecated `license_file` key with `license_files`
to fix setuptools deprecation warning:

> The license_file parameter is deprecated, use license_files instead.

Author: mgorny

setup.cfg

@@ -8,7 +8,7 @@ author = Holger Krekel, Oliver Bestwalter, Bernát Gábor and others
 maintainer = Bernát Gábor, Oliver Bestwalter, Anthony Sottile, Jürgen Gmach
 maintainer_email = gaborjbernat@gmail.com
 license = MIT
-license_file = LICENSE
+license_files = LICENSE
 platforms = any
 classifiers =
     Development Status :: 5 - Production/Stable