attempt to fix pypi publication

travis' dpl command seems to be broken for python 2:

travis-ci/dpl#1232

Use focal image and use python3

.travis.yml

@@ -1,4 +1,5 @@
 services: docker
+dist: focal
 
 env:
   global:
@@ -14,6 +15,7 @@ _python_template: &_python_template
   before_install: cd client/
   install: pip install -e .[test]
   script: py.test
+  before_deploy: sudo apt-get remove -y python2 && sudo ln -sf /usr/bin/python3 /usr/bin/python && python --version
   deploy:
     provider: pypi
     edge: true