Adopt uv for dependencies and CI

[trin94]

Hi again!

This moves the dev workflow over to uv.

What changed:

• Add a committed uv.lock and pin the dev tools.
• Run CI through uv (setup-uv + uv run tox), no more pip install.
• tox should install its envs from the lock, so the versions are reproducible.
• Drop the dead Travis config and move the badge to Actions.
• Remove the old nosetests target.
• Drop the unused ipython dependency.
• Tidy the makefile.
• Tidy the gitignore.
• Pin the taplo image.
• Run the full matrix even if one version fails.
• Bump the actions to their latest.
• Add Dependabot for the actions.

Left out of scope on purpose:

• Cleaning up the ruff findings / lint config.
• Switching to the uv build backend (it can't do git-tag versions yet, so hatchling + hatch-vcs stays).
• Dropping tox.

Thanks!

[ivankorobkov]

Impressive work, thank you.