Ah, I think I see what may have happened - when we finally switched all the stdlib importers over to PEP 302 (and consistently set __loader__), the pkgutil.get_loader fallback to pkgutil.find_loader stopped being exercised by the test suite (it's currently missing lower level unit tests), and so we missed that it was still calling a deprecated API. (And we don't currently have any CI set up to warn us when coverage of a module goes backwards)

I'll add some new tests to get coverage and change it to use importlib.util.find_spec instead.