Use of `GTK.TreeIter` Raises `DeprecationWarning`

[rmartin16]

Describe the bug

The SourceTreeModel triggers a deprecation warning.

/home/russell/github/beeware/briefcase/venv-3.10-briefcase/lib/python3.10/site-packages/toga_gtk/widgets/internal/sourcetreemodel.py:139: DeprecationWarning: Passing arguments to gi.types.Boxed.__init__() is deprecated. All arguments passed will be ignored.
  return False, Gtk.TreeIter(stamp=-1)

Steps to reproduce

briefcase dev or briefcase run linux system for the table or table_source example apps

Expected behavior

Deprecated functionality is not used.

Screenshots

No response

Environment

• Operating System: pop os 22.04
• Python version: 3.10.12
• Software versions:
  • Briefcase: 0.3.15.dev361+g02f412be
  • Toga: toga-gtk-0.3.2.dev383+g7d68e74d.d20230618

Logs

briefcase.2023_06_18-19_19_30.run.log

Additional context

No response