fix: type issues with Chart save methods.

[alec-bike]

Chart.save() methods trigger type-check warnings with ty. This PR adds some type annotations to avoid these warnings.

Resolves #3870.

To test run ty:

uvx ty check altair/vegalite/v6/api.py
uvx ty check altair/utils/save.py

uv run task test reveals no change in test results compared to main.

Notes:

• many more type issues still remain in these 2 files
• (reverted) replacing Union with | requires min python version 3.10

[alec-bike]

@mattijn could you please approve the workflow for the latest commit? The original commit failed due to incompatibility with python 3.9 so I reverted that change and rebased.

[mattijn]

Approved the workflow! Looks good! And great that you work on this PR🙌.

I've one question: do we need the change regarding this line:

Before

values: Any = Undefined

After

values: dict[str, Any] | list | InlineDataset | None = None

There have been long discussions in the past on the difference between None and Undefined (also type-wise) and at that time end up with the conclusion that there is a difference between these.

[alec-bike]

Thanks for approving, the PR passes now. Let me look more into Undefined vs None and revert or update as needed.

[alec-bike]

Neither ty nor basedpyright complained about the (original) use of Undefined so I reverted that change. Might be worth adding a unit test to illustrate the difference.

[mattijn]

Nice! Can this PR be moved out of draft status?

[alec-bike]

Done. PR is live and ready to review.

[mattijn]

Thanks @alec-bike! Looks great