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
25d0d2b5e8