Add python venv builder

[aaronbog]

This adds a builder that will create a .cvenv in the libraries folder.
Example usage:

builder |= pyvenv_builder(
        requirements_file_path= Path("./requirements.txt"),
        deps_folders= [Path("./deps/torta")],
        single_packages= ['pandas', 'matplotlib', 'seaborn', 'numpy']
    )

It also adds the cvenv bin folder to the path.
Allowing the command . activate
to start the Python venv.

This requires PR #28 to copy the deps_folders
and the requirement.txt file