get rid of waring in convert types function

cdplib/db_migration/MigrationCleaning.py

@@ -369,6 +369,7 @@ class MigrationCleaning:
                      
                     # might not be the smoothes solution but it works
                     python_type = str
+                    data = data.copy(deep=True)
                     data[column] = data[column].astype(python_type)
                     python_type = object
                     data[column] = data[column].astype(python_type)