Add PYTHONPATH environment variable to snapcraft.yaml

This fixes #1438: `encodings` module not found

The variable replaces the `sitecustomize.py` file from `snap/local` because it is no-longer functional due to changes in snapcraft.

snap/local/usr/lib/python3.8/sitecustomize.py

@@ -1,22 +0,0 @@
-import site
-import os
-
-snap_dir = os.getenv("SNAP")
-snapcraft_stage_dir = os.getenv("SNAPCRAFT_STAGE")
-snapcraft_part_install = os.getenv("SNAPCRAFT_PART_INSTALL")
-
-# Do not include snap_dir during builds as this will include
-# snapcraft's in-snap site directory.
-if snapcraft_stage_dir is not None and snapcraft_part_install is not None:
-    site_directories = [snapcraft_stage_dir, snapcraft_part_install]
-else:
-    superbuild_dir = os.path.join(snap_dir, 'odm/SuperBuild/install')
-    site_directories = [snap_dir, superbuild_dir]
-
-for d in site_directories:
-    if d:
-        site_dir = os.path.join(d, "lib/python3.8/site-packages")
-        site.addsitedir(site_dir)
-
-if snap_dir:
-    site.ENABLE_USER_SITE = False

snap/snapcraft.yaml

@@ -209,6 +209,7 @@ apps:
     environment:
       # Ensure libraries are found
       LD_LIBRARY_PATH: $SNAP/usr/lib/$SNAPCRAFT_ARCH_TRIPLET/blas:$SNAP/usr/lib/$SNAPCRAFT_ARCH_TRIPLET/lapack:$LD_LIBRARY_PATH
+      PYTHONPATH: $SNAP/odm/SuperBuild/install:$SNAP/lib/python3.8/site-packages:$SNAP/usr/lib/python3/dist-packages/:$SNAP/usr/lib/python3.8
     plugs:
       - home
       - network