general: try using HPI with astral-sh/uv

[karlicoss]

Not sure how it works with editable setup etc, but at least for CI it might give a significant speedup and possibly replace the hacky --parallel module install
related: #306

[karlicoss]

ok definitely worth it... not counting time to actually run tests (which is like 3-5 seconds)

tox -e tests-core --recreate 


without uv:
- first run (removed pip/tox cache) -- 21s
- second run (with all caches) -- 20s
- without --recretate -- 4.7s

with uv:
- first run (removed pip/tox/uv cache) -- 4.3s
- second run (with all caches) -- 2s
- without --recreate -- 1.8s

[karlicoss]

ok, really nice! typical times before were 3-4 minutes (although some runners, like 3.8 seem slower than other?), now it's like 1.5 minutes