KeyError: 'could not find foreign type Context'

[jwilk]

On Debian systems, when python3-gi-cairo is not installed, the error message you get is unhelpful:

$ sudo apt-get install python3-gi python3-cairo gir1.2-pango-1.0 python3-gi-cairo-
⋮

$ ubanner foo
Traceback (most recent call last):
  File ".../ubanner", line 129, in <module>
    main()
  File ".../ubanner", line 125, in main
    for line in lines:
  File ".../ubanner", line 41, in pango_render
    layout = PangoCairo.create_layout(ctx)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
KeyError: 'could not find foreign type Context'

What's going on here?