feat: use python-discovery

[henryiii]

This enables version ranges to be used. It also can find Pythons from a variety of sources that we didn't support before.

• Version checking can be slow.
  • We check classic name-based discovery first, so should have no impact unless it's used
  • This currently uses a disk cache - maybe we should not use it, though? Where should we put it? (doesn't seem like tox and virtualenv use it?)
• We should also support script python-requires using this

🤖 Assisted-by: OpenCode:Kimi-K2.5