refactor: build on ubuntu focal using python 3.8

Author: azubieta

AppImageBuilder.yml

@@ -28,7 +28,8 @@ AppDir:
   apt:
     arch: amd64
     sources:
-      - sourceline: 'deb [arch=amd64] http://archive.ubuntu.com/ubuntu/ bionic main restricted universe multiverse'
+      - sourceline: 'deb [arch=amd64] http://archive.ubuntu.com/ubuntu/ focal main restricted universe multiverse'
+      - sourceline: 'deb [arch=amd64] http://archive.ubuntu.com/ubuntu/ focal-updates main restricted universe multiverse'
         key_url: 'http://keyserver.ubuntu.com/pks/lookup?op=get&search=0x3b4fe6acc0b21f32'
 
     include:
@@ -45,7 +46,6 @@ AppDir:
     exclude: []
 
   after_bundle: |
-    export PYTHONPATH=${APPDIR}/usr/lib/python3.8/site-packages
     python3.8 -m pip install --ignore-installed --prefix=/usr --root=$APPDIR  install .
 
   runtime: