Use new key to specify license file in setup.cfg

to prevent deprecation warning.

Author: CarstenGrohmann

setup.cfg

@@ -6,7 +6,7 @@ long_description = file:README.rst
 long_description_content_type = text/x-rst
 author = Philippe Pepiot
 author_email = phil@philpep.org
-license_file = LICENSE
+license_files = LICENSE
 classifiers =
     Development Status :: 5 - Production/Stable
     Environment :: Console