Support for cleaning up Python `venv`s about ocy

[alexd6631]

Thanks for your feedback @virtualritz,

Ocy is based on quite rudimentary matcher system, basically it will scan for witness files such as 'package.json' or 'Cargo.toml' and perform a clean action such as removing a conventional target dir (node_modules, target) or perform a shell command.

I am afraid there is no clear convention on python projects to apply such logic, but as I am only writing python occasionally, I would love to hear if you have ideas to exploit the project structure.