Switch to license_files in setup.cfg

https://setuptools.pypa.io/en/latest/userguide/declarative_config.html

Author: amureki

setup.cfg

@@ -5,7 +5,7 @@ author_email = [email protected]
 description = The lean workflow automation framework for machines with heart
 long_description = file: README.rst
 license = BSD
-license_file = LICENSE
+license_files = LICENSE
 url = https://github.com/codingjoe/joeflow
 # Find more classifiers here:
 # https://pypi.org/pypi?%3Aaction=list_classifiers