[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 scikit-hep/uhi#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:

• tested changes locally
• updated the docs (if necessary)

[github-actions]

Test Results

    22 files      22 suites   3d 7h 41m 18s ⏱️
 3 702 tests  3 289 ✅   0 💤   413 ❌
79 493 runs  72 105 ✅ 840 💤 6 548 ❌

For more details on these failures, see this check.

Results for commit 6619eba.