shutil.rmtree's error callback was renamed onerror -> onexc in 3.12 and the
two receive different third arguments, so sandbox_destroy raised TypeError on
every 3.11 run despite requires-python = ">=3.11". Route both through one
helper and test it.
Found by running the suite under 3.11 before publishing; the whole suite now
passes on 3.11 and 3.13.
Ryan Malloy
1103b7649f