Dialect: Fix get_pk_constraint to return list instead of set type

src/sqlalchemy_cratedb/dialect.py

@@ -326,7 +326,7 @@ def result_fun(result):
             (table_name, schema or self.default_schema_name)
         )
         pks = result_fun(pk_result)
-        return {'constrained_columns': pks,
+        return {'constrained_columns': list(pks),
                 'name': 'PRIMARY KEY'}
 
     @reflection.cache