GTK Canvas doesn't set font description correctly

[rmartin16]

Describe the bug

Canvas crashes with TypeError on Linux.

Traceback (most recent call last):
  File "/home/russell/github/beeware/toga/examples/canvas/build/canvas/pop/jammy/canvas-0.0.1/usr/lib/canvas/app_packages/toga_gtk/app.py", line 98, in gtk_startup
    self.interface.startup()
  File "/home/russell/github/beeware/toga/examples/canvas/build/canvas/pop/jammy/canvas-0.0.1/usr/lib/canvas/app/canvas/app.py", line 158, in startup
    self.render_drawing()
  File "/home/russell/github/beeware/toga/examples/canvas/build/canvas/pop/jammy/canvas-0.0.1/usr/lib/canvas/app/canvas/app.py", line 378, in render_drawing
    self.draw_shape(context)
  File "/home/russell/github/beeware/toga/examples/canvas/build/canvas/pop/jammy/canvas-0.0.1/usr/lib/canvas/app/canvas/app.py", line 388, in draw_shape
    drawing_instructions(context, factor)
  File "/home/russell/github/beeware/toga/examples/canvas/build/canvas/pop/jammy/canvas-0.0.1/usr/lib/canvas/app/canvas/app.py", line 509, in draw_instructions
    width, height = self.canvas.measure_text(text, font, tight=True)
  File "/home/russell/github/beeware/toga/examples/canvas/build/canvas/pop/jammy/canvas-0.0.1/usr/lib/canvas/app_packages/toga/widgets/canvas.py", line 748, in measure_text
    return self._impl.measure_text(text, font, tight=tight)
  File "/home/russell/github/beeware/toga/examples/canvas/build/canvas/pop/jammy/canvas-0.0.1/usr/lib/canvas/app_packages/toga_gtk/widgets/canvas.py", line 232, in measure_text
    layout.set_font_description(self.native)
TypeError: argument desc: Expected Pango.FontDescription, but got gi.repository.Gtk.DrawingArea

Steps to reproduce

Run briefcase run linux system for the canvas or tutorial4 examples.

Expected behavior

Does not crash.

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.dev380+g7408a792.d20230618

Logs

briefcase.2023_06_18-16_23_56.run.log

Additional context

No response