build: Starlarkify python flags

[mai93]

Add starlark flags for --python_path, --build_python_zip and --incompatible_default_to_explicit_init_py.

• The transitions logic is updated to set both the native and starlark versions of the flags to allow alternating between them until the native ones are removed.
• --build_python_zip is changed to boolean instead of Tristate with default value set to True on windows and False otherwise.
• scope = universal attribute is added to the starlark flags so they can be propagated to exec config on Bazel 9. This required upgrading bazel_skylib version to have scope attribute defined.

Work towards: #3252

cc @gregestren

[dougthor42]

One gazelle-specific comment, otherwise the Gazelle stuff LGTM.

I'll leave final approval to Richard or Aignas

[aignas]

Could you please add a note in Changes section of the changelog to note that bazel_skylib has been upgraded to 1.8.2? See the previous mentions for bazel_skylib for an example.

[rickeylev]

impl mostly looks good, though the logic for checking python_path i'm not sure of. Various nits about project-local styles.

[rickeylev]

@gemini-code-assist /review

[gemini-code-assist]

Code Review

This pull request continues the effort to migrate native Python flags to Starlark flags, which is a great improvement for the maintainability and future-proofing of rules_python. The changes introduce Starlark flags for --python_path, --build_python_zip, and --incompatible_default_to_explicit_init_py, and correctly update the transition logic to support both native and Starlark versions during the migration period. The upgrade of bazel_skylib to support the scope = universal attribute is also a necessary and well-executed change.

The implementation is solid and covers all the required aspects, including updating tests and examples. I have a couple of minor suggestions to improve the usability of the new flags by adding help text and making an error message more descriptive. Overall, this is a high-quality contribution.