This was discussed a bit on Sunday's thread[2], and my understanding is that Maven's ability to use PGP in this way is effectively due to Sonatype assuming a large amount of operational and maintenance burden. PyPI doesn't have those kind of resources available to it. Even assuming that the service was gifted that kind of support, it would still cause a lot of heartburn with existing signatures and carry forwards all of the legacy baggage of PGP that we're trying to eliminate entirely.
[1]: https://gist.github.com/rjhansen/67ab921ffb4084c865b3618d695...