[Draft][Python][UHI] Start introducing UHI serialization

[siliataider]

This Pull request:

This PR introduces the first steps toward enabling UHI serialization for ROOT histograms (see https://github.com/scikit-hep/uhi/pull/167).

This works by implementing the _to_uhi_ method for ROOT histograms: it provides an Intermediate Representation of the histogram described by the UHI schema.

[!WARNING]
This is not a feature-complete implementation. Only the essential components to serialize ROOT histograms are included, with the rest to come in future PRs.

To-do before review

• [ ] add _from_uhi_ to convert from a UHI histogram to a ROOT histogram
• [ ] add tests
• [ ] update the docs

Checklist:

• [x] tested changes locally
• [ ] updated the docs (if necessary)

[github-actions[bot]]

Test Results

    21 files      21 suites   3d 17h 50m 3s ⏱️  3 794 tests  3 792 ✅   0 💤 2 ❌ 77 834 runs  77 638 ✅ 192 💤 4 ❌

For more details on these failures, see this check.

Results for commit 3adad1ba.

:recycle: This comment has been updated with latest results.