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The saved_test_environment context manager should check that sys.path_hooks, sys.path_importer_cache, and __import__ have not changed.
The thing that is tricky, though, is that sys.path_importer_cache is legitimately mutated by other tests simply because valid imports put in new values. The most conservative check, then, is to validate that pre-existing keys do not change their values. A more liberal check is to whitelist finders and validate that no key have a value that is not on the whitelist.
And for __import__, assigning against __builtins__.__import__ should be enough to also catch builtins.__import__.