Fix potential free of null pointer in stringlist_delete - #435

cibuildwheel: Invalid skip selector: 'pp*'. This selector matches a group that wasn't enabled. Enable it using the `enable` option or remove this selector. This selector will have no effect.

cibuildwheel: Invalid skip selector: 'cp37*'. cibuildwheel 3.x no longer supports Python < 3.8. Please use the 2.x series or update `skip`. This selector will have no effect.