Unlike the other bindings (Lua, JS, Vala) where we only have a demo
plug-in, we actually have a bunch of interesting and useful Python
plug-ins.
Furthermore, we are running several Python scripts through GIMP during
our build (to generate a few images), so pygobject becomes a build
dependency anyway, even if it may not be a run dependency with
-Dpython=disabled.
This all becomes a bit confusing and in fact handling this case (of not
installing Python plug-ins) seems like an annoyance while we lose good
core plug-ins. So let's just make Python plug-ins mandatory. It's not
like Python is very controversial these days, and AFAWK, it is available
on every platform out there.
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