Fix mypy

Author: mmwinther

src/dapla_metadata/datasets/core.py

@@ -575,7 +575,9 @@ def copy_variable(
 
         source_variable = source_variables_lookup[source_short_name]
         # Always override the data type to ensure it matches the physical dataset.
-        source_variable.data_type = self.variables_lookup[target_short_name].data_type
+        source_variable.data_type = self.variables_lookup[  # type: ignore[assignment]
+            target_short_name
+        ].data_type
         self.variables_lookup[target_short_name] = source_variable
 
         source_variable = all_optional_model.Variable.model_validate(source_variable)