PR Summary

Changes such as #12885 are actually backcompat-breaking (in a strict sense) because calls of the form FormLayout(data, comment, parent) have changed their meaning (after that PR, the third argument is interpreted as the with_margin arg).

In practice, there's no reason for formlayout to be public (the official version is PyPI-installable), so let's make it private.

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way