Always use python3 during molecule testing

Python2 went out of style more than a decade ago, and was "sunset"
over four years ago, 2020-01-01.

Sure CentOS might have some additional python2 support thanks to
RedHat, but you know what else it has? Python3. And with
epel-release, we get access to python3-PyMySQL, which allows us to
keep things simple.

molecule/zabbix_proxy/molecule.yml

@@ -22,10 +22,8 @@ provisioner:
     ANSIBLE_ROLES_PATH: $HOME/.ansible/collections/ansible_collections/community/zabbix/roles
   inventory:
     group_vars:
-      python3:
+      all:
         ansible_python_interpreter: /usr/bin/python3
-      python:
-        ansible_python_interpreter: /usr/bin/python
       v64:
         zabbix_proxy_version: 6.4
       v62: