pygtk apps fail with gnome-3-34 extension

For reference: https://forum.snapcraft.io/t/gnome-3-34-extension-and-python-segfault/19650

The gnome-3-34-1804 content snap includes a python3 interpreter. For snaps that build python parts there is another python3 interpreter bundled as well. At runtime the bundled interpreter doesn't seem to be compatible with the version of python the modules were built with.

This can be worked around with a combination of build-environment and environment. Here's an excerpt:

apps:
  app:
    environment:
      PATH: $SNAP_DESKTOP_RUNTIME/usr/bin:$PATH
      PYTHONPATH: $PYTHONPATH:$SNAP_DESKTOP_RUNTIME/usr/lib/python3.6/site-packages:$SNAP/usr/lib/python3.6/site-packages:$SNAP/lib/python3.6/site-packages
    ...

parts:
  part:
    plugin: meson
    source: .
    build-environment:
      - PATH: /snap/gnome-3-34-1804-sdk/current/usr/bin:$PATH
    ...