[202012][platform/barefoot] (#8543)

Why I did it
Pcied running by python 2.

How I did it
dropped python2 support and add python3 support for pcied in file docker-pmon.supervisord.conf.j2

How to verify it
docker exec pmon supervisorctl status

dockers/docker-platform-monitor/docker-pmon.supervisord.conf.j2

@@ -150,7 +150,7 @@ dependent_startup_wait_for=rsyslogd:running
 
 {% if not skip_pcied %}
 [program:pcied]
-command=/usr/local/bin/pcied
+command={% if API_VERSION == 3 and 'pcied' not in python2_daemons %}python3 {% else %} python2 {% endif %}/usr/local/bin/pcied
 priority=10
 autostart=false
 autorestart=unexpected