fix(setup.cfg): use license_files over license_file
license_file is deprecated:
```
/usr/lib/python3.11/site-packages/setuptools/config/setupcfg.py:463: SetuptoolsDeprecationWarning: The license_file parameter is deprecated, use license_files instead.
warnings.warn(msg, warning_class)
```
@@ -7,7 +7,7 @@ url = https://github.com/packit/packit
author = Red Hat
author_email = user-cont-team@redhat.com
license = MIT
-license_file = LICENSE
+license_files = LICENSE
classifiers =
Development Status :: 4 - Beta
Environment :: Console
