Fix: add an assertion in throttling.py to please mypy 1.14

We are currently pinning mypy 1.13.0, but even if mypy<1.14 does not
complain without this assertion, it should. So let's fix it right now.

swh/web/api/throttling.py

@@ -132,6 +132,7 @@ class SwhWebRateThrottle(ScopedRateThrottle):
             )
             self.scope = default_scope
 
+        assert default_scope is not None
         exempted_networks = self.get_exempted_networks(default_scope)
         exempted_ip = False