Use inventory plugin instead of script for Python dynamic inventory example

See: [Developing dynamic inventory](https://docs.ansible.com/ansible/latest/dev_guide/developing_inventory.html).

Scripts are now actually executed using the 'script' inventory plugin. It's preferred nowadays to write an inventory plugin to use dynamic inventory rather than writing an inventory script (unless you want to maintain the script in a non-Python language).

So maybe keep the PHP-based script example, but change the Python one to an inventory plugin.