[3.12] gh-116307: Proper fix for 'mod' leaking across importlib tests (GH-116680) (#116684)

gh-116307: Proper fix for 'mod' leaking across importlib tests (GH-116680)
(cherry picked from commit a2548077614f81f25a2c3465dabb7a0a3885c40c)


gh-116307: Create a new import helper 'isolated modules' and use that instead of 'Clean Import' to ensure that tests from importlib_resources don't leave modules in sys.modules.

Co-authored-by: Jason R. Coombs <jaraco@jaraco.com>