python_pip says it supportge purge, but doesn't implement it

[Ken Robertson]

The pip resource exposes a :purge action, however the provider implementation doesn't actually implement it. When attempting to use purge, end up with an error:

NoMethodError: No resource or method named `action_purge' for `Chef::Provider::PythonPip ""'

[Noah Kantrowitz]

This is now explicitly unsupported in https://github.com/poise/poise-python with appropriate docs.