ci: add downstream tests for `marimo`

[AlessandroMiola]

What type of PR is this? (check all applicable)

• [ ] 💾 Refactor
• [ ] ✨ Feature
• [ ] 🐛 Bug Fix
• [ ] 🔧 Optimization
• [ ] 📝 Documentation
• [x] ✅ Test
• [ ] 🐳 Other

Related issues

• Closes #1163

Checklist

• [x] Code follows style guide (ruff)
• [ ] Tests added
• [ ] Documented the changes

If you have comments or can explain your changes, please do so below.

Attempt at adding downstream tests for marimo to the CI pipeline.

Notes:

• marimo's full test suite also includes additional tests for frontend/cli etc.; I've omitted them as not relevant to the python backend testing functionality.
• marimo leverages on hatch, though pyproject.toml shows the following

  [tool.hatch] installer = "uv"

• also, test:test, test-optional:test and typecheck:check do refer to the following pytest and mypy commands

  [tool.hatch.envs.test.scripts] test = "pytest{env:HATCH_TEST_ARGS:} {args:tests}" ... [tool.hatch.envs.test-optional] template = "test" ... [tool.hatch.envs.typecheck.scripts] check = "mypy marimo/"

All this said, I've kept the commands as-is (trying to align with the approach in https://github.com/narwhals-dev/narwhals/pull/1161, even though I haven't referenced make commands as they do hardcode the python version in within).

Please let me know your thoughts on these points so that I can tweak accordingly :) (assuming that everything works as intended 🥶 ---EDIT: it does not yet)