doc: python: refreshing virtualenv section for venv

Updating section about imperative use of ad-hoc virtual-environments for
use of pythons built-in `venv` module via venvShellHook.  Also trying to
make it a bit friendlier to beginners by adding a bit more explanation
to the code snippet and some remarks old-school virtualenv.

Adjusting for venvShellHook and adding manual example

Adding pip install and replacing python2 example with python3