Prepare v3.1.0 release

[lognaturel]

Prepares for a 3.1.0 release off of a release branch.

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

We have found that #768 does affect some forms. We think #746 may still have subtle regressions. So to make client_editable available with minimal risk, we're releasing it on the 3.x line.

We also considered reverting #768 but we want to take some time to understand it better first.

What are the regression risks?

Very low -- this is a purely additive change that is easy to inspect.

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

No.

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