Adopt uv for dev + CI install

[martinkersner]

Closes #100

Summary

Speed up contributor setup and CI by using uv for installs, keeping the existing requirements/*.txt layout (no deps moved into pyproject.toml, no committed uv.lock).

• CI (.github/workflows/python-package.yml): add astral-sh/setup-uv@v6 (with enable-cache: true) and replace the pip install step with uv pip install --system -r requirements/requirements-test.txt. Dropped the now-redundant actions/cache@v4 pip-cache step (uv provides its own cache). Matrix, black, flake8, and the offline mocked-test step are unchanged.
• Docs (README.md): Local Development + Tests now use uv venv / uv pip install / uv run pytest, with a link to the uv docs. Integration tests still require DATAMAXI_API_KEY.

Note: tox was already removed upstream (#98), so nothing to do there.

Test plan

• CI will validate the uv-based install across the py3.10–3.14 matrix (black + flake8 + offline mocked tests).
• Live integration tests still run locally only (DATAMAXI_API_KEY=... pytest tests/); not run in CI.
• Verified locally: workflow YAML parses (yaml.safe_load), no remaining tox references, only .github/workflows/python-package.yml and README.md changed (no .py edits, so black/flake8 unaffected).