Okay, I've tracked it down to the new importlib.readers.ZipReader class keeping the zip file open, presumably until it gets GC'd. This is used by certifi to extract the CA certs from the whl when ensurepip is doing the self-install from the mounted wheel.

Jason is already on this bug, which is convenient :)

I haven't yet figured out whether there's a convenient way for the reader to not keep the ZIP open for as long as it exists, but I think that's going to be the safest fix.

We should definitely fix this one ourselves without forcing users to make changes to accommodate. As I mentioned above, it's only in 3.10 right now, but it's blocking updated pip and setuptools versions downlevel.