[Spikhalskiy]

Python version transitions also reach native dependencies that don't use Python, so a C++ or Rust library under bindings for several Python versions is configured once per version. Add a root-module config.python_version_scope(scope = "project") tag that gives //python/config_settings:python_version Bazel's project scope. Ownership is delegated to the consumer's //:PROJECT.scl through an actual alias, so targets outside it see the command-line version and share one configuration. Universal scope stays the default, and tags from dependency modules are ignored.

Project scope requires Bazel 9.2 or later, the first release that keeps project-scoped flags across exec transitions (bazelbuild/bazel#28320). Owned exec tools such as the precompiler keep the selected version, which preserves the requirement behind #4335. Older Bazel versions fail with an explicit error.

aspect_rules_py terminals set this flag as well as their own; the matching opt-in there is aspect-build/rules_py#1650.

An integration test covers target and exec propagation, convergence of the native dependency's configuration, the unchanged default, and ignored dependency-module tags.

Fixes #4375

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[Spikhalskiy]

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