Merge pull request #13924 from larrybradley/license-files

Use license_files instead of deprecated license_file in setup

setup.cfg

@@ -3,7 +3,7 @@ name = astropy
 author = The Astropy Developers
 author_email = astropy.team@gmail.com
 license = BSD-3-Clause
-license_file = LICENSE.rst
+license_files = LICENSE.rst
 url = http://astropy.org
 project_urls =
     Documentation = https://docs.astropy.org