[3.13] gh-144601: Use `_testmultiphase` instead of `_testsinglephase` in `test_importlib` (GH-144769) (GH-144877)

[3.14] gh-144601: Use `_testmultiphase` instead of `_testsinglephase` in `test_importlib` (GH-144769)
(cherry picked from commit d1486623263adea6a5138453bacad36a50c14e95)

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>