Rebase to core20

Misc updates to support rebase of snap on core20:
 - Python plugin is py3 only.
 - No need to massage site.py.
 - Correct path to python3 binary.

Change-Id: I979e6a0e8eab9caaf313f232c6eaf78245157a5b

snapcraft.yaml

@@ -14,7 +14,7 @@ description: |
   project specific command-line clients.
 confinement: strict
 grade: stable
-base: core18
+base: core20
 environment:
   REQUESTS_CA_BUNDLE: /etc/ssl/certs/ca-certificates.crt
   PYTHONIOENCODING: utf-8
@@ -149,7 +149,6 @@ apps:
 parts:
   openstackclients:
     plugin: python
-    python-version: python3
     python-packages:
       - aodhclient
       - osc-placement
@@ -198,11 +197,8 @@ parts:
       - cargo
     override-prime: |
       snapcraftctl prime
-      # Now that everything is built, let's disable user site-packages
-      # as stated in PEP-0370
-      sed -i usr/lib/python3.6/site.py -e 's/^ENABLE_USER_SITE = None$/ENABLE_USER_SITE = False/'
       # This is the last step, let's now compile all our pyc files.
-      ./usr/bin/python3 -m compileall .
+      ./bin/python3 -m compileall .
       # Generate completer script snippets
       mkdir -p usr/share/bash-completion/completions
       ./bin/openstack complete > usr/share/bash-completion/completions/openstack