setup.py: use correct license_expression (#1345)

Author: prandla

setup.py

@@ -194,7 +194,7 @@ class build_py_and_l10n(build_py):
         ],
     },
     keywords="ioi programming contest grader management system",
-    license_expression="AGPL-3.0-only",
+    license_expression="AGPL-3.0-or-later",
     classifiers=[
         "Development Status :: 5 - Production/Stable",
         "Natural Language :: English",