Replace setup.py with pyproject.toml, add CI

[chrieke]

Packaging cleanup, no library code changes. Stacked on #14 (which adds the tests CI runs), so review that one first.

• setup.py -> pyproject.toml, drops the root requirements.txt that it read deps from
• metadata was stale: no classifiers at all, python_requires >=3.6, streamlit>=0.79
• now requires-python >=3.10, classifiers up to 3.12, and a dev extra for pytest/ruff
• ruff for both formatting and linting, github action running it plus pytest on 3.10 and 3.12
• ruff sorted the imports in the examples, that's the only change there

Rules are pinned to E4/E7/E9/F/I instead of ruff's defaults, those keep growing between releases and would turn CI red on unrelated PRs.

Not on 3.13: keplergl 0.3.7 is sdist only and build-requires pyarrow~=16, which has no 3.13 wheels. Its build deps also still import pkg_resources, hence the setuptools<81 constraint in the workflow.

Version stays at 0.3.0, no release here.