license_file -> license_files to make pip happy (#992)

https://setuptools.pypa.io/en/latest/history.html#id522

license_file has been deprecated

Co-authored-by: Ian Stapleton Cordasco <graffatcolmingov@gmail.com>

Author: mcepl

setup.cfg

@@ -1,5 +1,5 @@
 [metadata]
-license_file = LICENSE
+license_files = LICENSE
 name = twine
 author = Donald Stufft and individual contributors
 author_email = donald@stufft.io