I believe OpenDev (OpenStack Infra) just rewrites the URLs
While technically correct, what you’re seeing there is simply Apache mod_proxy coupled with mod_cache to minimize the number of requests made by CI test nodes to PyPI over the Internet in the various public cloud regions where we have donated resources. For those curious, the Apache configuration for that proxy can be found here: Making sure you're not a bot!
(Ignore the “wheel” rewrites below there, that’s for a separate auxiliary index mixed in via --extra-index-url to backfill platform-specific wheels on old releases of projects that only supplied sdists, we mostly don’t use it any more and have deprecated but not removed it yet.)
Once upon a time we did mirror PyPI through a variety of means, mostly with bandersnatch once it existed, but the storage cost of maintaining a complete mirror became overwhelming when AI-oriented projects started uploading massive nightly dataset builds and the size of the mirror exploded by an order of magnitude or more in a matter of a few months.