Make python meson template runnable from builddir

This change makes it possible to run your python project from your build directory.

I encountered this problem first when I added a dependency to my project and didn't want to bother with flatpak's too complicated way of dealing with pip dependencies.

I have this change in my project but I couldn't test this MR because gnome-builder crashed when running gnome-builder in gnome-builder and when I wanted to export the flatpak bundle.