use license_files instead of the deprecated license_file

Change-Id: I0d3716d87ccc5c18c5e944e71f986df437a1f62a

setup.cfg

diff --git a/setup.cfg b/setup.cfg
index a824e19..f147e52 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -8,7 +8,7 @@
 author = Mike Bayer
 author_email = mike@zzzcomputing.com
 license = MIT
-license_file = LICENSE
+license_files = LICENSE
 classifiers =
     Development Status :: 5 - Production/Stable
     License :: OSI Approved :: MIT License