chore: Use tox-uv to ease lower bound checking, test on Python 3.13

[effigies]

This PR does the following:

• Adopts tox-uv as a speedup for constructing test environments and automatically calculating minimum versions.
  • This requires setting lower bounds on test dependencies, which is fine. We don't need to be very conservative with these. I just picked some that have worked with other projects I've updated recently and the most recent major versions of some others.
• Adds the scipy nightly build package index so we'll see breakage coming with plenty of warning. Deferred.
• Begins testing on 3.13, which is enabled because altair released a new minor version. This cherry-picks @Remi-Gau's [MAINT] support python 3.13 #1093.
• Some cleanups to coverage. I added tests/ to coverage, even though it's currently only data, so that they'll get picked up if we ever add tests that we don't want installed there.

[effigies]

Well, the good news is that the pre-release tests are now finding nightly packages.

[effigies]

Okay, deferring nightly packages to another PR, because this is already an infrastructure improvement.

I also want to consolidate the package upload step into this workflow, since we're now building it with attestations. I can do that in this PR or another. LMK.

[effigies]

Nevermind. I want to do that one separately as well, and it's orthogonal to this.

This is ready for review.

[effigies]

Following the principle of no objections for 1 week and it still looks good to me, I'm merging.