Update setup.cfg to description_file field (#7248)

Resolve `UserWarning: Usage of dash-separated 'description-file' will not be supported in future versions. Please use the underscore name 'description_file' instead`

Author: glenn-jocher

setup.cfg

@@ -4,7 +4,7 @@
 
 [metadata]
 license_file = LICENSE
-description-file = README.md
+description_file = README.md
 
 
 [tool:pytest]