Rewrite long strings using \ instead of + (concatenation).

bfxapi/tests/test_websocket_serializers.py

@@ -10,11 +10,12 @@ class TestWebsocketSerializers(unittest.TestCase):
     def test_websocket_serializers(self):
         for serializer in map(serializers.__dict__.get, serializers.__serializers__):
             self.assertTrue(issubclass(serializer.klass, _Type),
-                f"_Serializer <{serializer.name}>: .klass field must be a subclass of _Type (got {serializer.klass}).")
+                f"_Serializer <{serializer.name}>: .klass field must be a subclass " \
+                    f"of _Type (got {serializer.klass}).")
 
             self.assertListEqual(serializer.get_labels(), list(serializer.klass.__annotations__),
-                f"_Serializer <{serializer.name}> and _Type <{serializer.klass.__name__}> "
-                    + "must have matching labels and fields.")
+                f"_Serializer <{serializer.name}> and _Type <{serializer.klass.__name__}> " \
+                    "must have matching labels and fields.")
 
 if __name__ == "__main__":
     unittest.main()