fix for sonarcloud-reported bug (#1945)

invalid "+" operation between incompatible types (str and type) - [sonarcloud-reported bug](https://sonarcloud.io/project/issues?issues=AYDHhHV9L8VDMGmTmRtc&open=AYDHhHV9L8VDMGmTmRtc&id=RDFLib_rdflib)

Changed exception string construction to use `f""` format

rdflib/plugins/serializers/trix.py

@@ -47,7 +47,7 @@ def serialize(
         elif isinstance(self.store, Graph):
             self._writeGraph(self.store)
         else:
-            raise Exception("Unknown graph type: " + type(self.store))
+            raise Exception(f"Unknown graph type: {type(self.store)}")
 
         self.writer.pop()
         stream.write("\n".encode("latin-1"))