refactor: Centralize Vega project versioning

[dangotbanned]

Description

This PR seeks to simplify version bumps of projects we depend on within the https://github.com/vega org.

The new process I'm proposing is documented in https://github.com/vega/altair/blob/5aec51ef8eaf345936436abddd903ea87b86b968/NOTES_FOR_MAINTAINERS.md.
In short, we bump versions in pyproject.toml and they propagate to everywhere they were previously manually maintained:

altair/pyproject.toml

Lines 103 to 110 in 5aec51e

	[tool.altair.vega]
	# Minimum/exact versions, for projects under the `vega` organization
	vega-datasets = "v2.11.0" # https://github.com/vega/vega-datasets
	vega-embed = "6" # https://github.com/vega/vega-embed
	vega-lite = "v5.20.1" # https://github.com/vega/vega-lite
	vegafusion = "1.5.0" # https://github.com/vega/vegafusion
	vl-convert-python = "1.7.0" # https://github.com/vega/vl-convert

The new module https://github.com/vega/altair/blob/5aec51ef8eaf345936436abddd903ea87b86b968/tools/versioning.py handles said propagation.

Motivation

Vega-Lite

I think it was unintentional, but we missed a vega-lite bump in https://github.com/vega/altair/releases/tag/v5.5.0.
We're still on https://github.com/vega/vega-lite/releases/tag/v5.20.1 but we should be on https://github.com/vega/vega-lite/releases/tag/v5.21.0 and now the latest is https://github.com/vega/vega-lite/releases/tag/v5.23.0

Additionally, these appear to be out-of-sync?

vl-convert

altair/pyproject.toml

Lines 56 to 58 in 1208c5d

	[project.optional-dependencies]
	save = [
	"vl-convert-python>=1.7.0",

altair/altair/utils/_importers.py

Lines 47 to 48 in 1208c5d

	def import_vl_convert() -> ModuleType:
	min_version = "1.6.0"

VegaFusion

altair/pyproject.toml

Line 66 in 1208c5d

"vegafusion[embed]>=1.6.6",

altair/altair/utils/_importers.py

Lines 12 to 13 in 1208c5d

	def import_vegafusion() -> ModuleType:
	min_version = "1.5.0"

Future

I'm planning to pin vega-datasets for (#3631).

There are also likely to be a lot of version changes coming up as a result of (vega/vega#3990).
IMO having a single, highly visible place to update will make this process hopefully painless for us

Tasks

• Work-around for tomllib when sys.version_info < (3, 11)
  • See (https://github.com/vega/altair/actions/runs/12456841158/job/34770747096?pr=3720)
• Update RELEASING.md (dc0215c)
• Update NOTES_FOR_MAINTAINERS.md
  • Mostly finished in (c45f1c4)
  • Remove VegaFusion section (thread)