[Python] How to use a custom font in a GTK app

I have a font file /path/to/app/fonts/custom-font.ttf and I want to use it. How do you import a custom TTF for use in a GTK+3.0 app?

from gi.repository import Gtk, Pango

# ...

lbl = Gtk.Label()
lbl.modify_font(Pango.FontDescription("sans 48"))
# lbl.modify_font(Pango.FontDescription("custom-font 48"))

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