Prepare v4.3.0 release

[lognaturel]

Why is this the best possible solution? Were any other approaches considered?

Followed https://github.com/XLSForm/pyxform?tab=readme-ov-file#releasing-pyxform

What are the regression risks?

The major change in this release is Entity-related and isolated to Entities with very low risk to other functionality. The other significant change is that we no longer warn for groups without labels. This could possibly be disruptive but we can't really know without releasing. The PR has been merged for a while without comment.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

Before submitting this PR, please make sure you have:

• included test cases for core behavior and edge cases in tests
• run python -m unittest and verified all tests pass
• run ruff format pyxform tests and ruff check pyxform tests to lint code
• verified that any code or assets from external sources are properly credited in comments

[sadiqkhoja]

Verified that

• there is no new release of ODK Validate
• tests with ODK_Validate=true are passing
• CHANGE.txt contains details of all merged PRs
• pyproject.toml, pyxform/__init__.py are updated