Use license_files instead of license_file

This addresses a warning that shows up while making a release

setup.cfg

@@ -11,7 +11,7 @@ long_description = file: README.md
 long_description_content_type = text/markdown
 maintainer = "Alex Conrad"
 maintainer_email = "[email protected]"
-license_file = LICENSE
+license_files = LICENSE
 keywords = "cobertura coverage diff report parser parse xml"
 url = https://github.com/aconrad/pycobertura
 classifiers =