Make dev and ci environments use uv

[eivindjahren]

Also changes the dependabot schedule to quarterly.

[Copilot]

Pull request overview

This PR migrates the project’s dev/CI dependency management from pip + requirements.txt to uv, and updates Dependabot to run on a quarterly cadence.

Changes:

• Add uv.lock and define a dev dependency group in pyproject.toml.
• Update GitHub Actions workflow to install/sync dependencies via uv and run tooling through uv run.
• Update documentation (README) and Dependabot config to match the new dependency workflow and cadence.

Reviewed changes

Copilot reviewed 4 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
uv.lock	Adds an uv lockfile for reproducible dev/CI installs.
requirements.txt	Removes legacy pip requirements file in favor of uv groups/lockfile.
README.md	Updates test instructions to use uv sync and uv run.
pyproject.toml	Introduces [dependency-groups] for dev tools and configures [tool.uv].
.github/workflows/build.yml	Switches CI environment setup/tests/style checks to use uv.
.github/dependabot.yml	Moves to quarterly updates and adds uv lockfile updates.

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