Re unit tests. Doh. Of *course* there are tests for importing from zipfiles. And I just ran them (on Windows) and they worked. So there's something more complex going on here and I don't know how you test for it, sorry (short of by trying an import from the pip wheel that's bundled as part of _ensurepip - that's ugly and not very focused, but on the other hand it does test the precise use case that I have here...)