Use the GLib.MAXINT32 instead of GObject.G_MAXINT32

G_MIN/MAX constants had been moved into GObject overrides
(See pygobject c2aa6f0d0ed4c4e60f081b106dc7a65513963fce) but
it is recommended to use the constants from GLib as those
are pulled in directly from GI.

Acked-by: Manuel Quiñones <manuq@laptop.org>