Set the license_files tag. (#1082)

setup.cfg

@@ -29,6 +29,7 @@ project_urls =
     Repository=https://www.dulwich.io/code/
     GitHub=https://github.com/dulwich/dulwich
     Bug Tracker=https://github.com/dulwich/dulwich/issues
+license_files = COPYING
 
 [options.extras_require]
 fastimport = fastimport