Revisit "nested virtualenvs" problem

[facundobatista]

At some point in time, with some Python versions and ways to handle environments, if you created a virtualenv being already inside another virtualenv, that creation was broken.

To avoid this situation biting users, we added a couple of checks that will alert early that the situation was there.

Fast forward several years. Today it may work. We need to check if this is the case.

If not, everything remain as before.

If now it's possible to have a virtualenv created while you were in other virtualenv, then:

• check the README and verify if we're saying something about this, and fix
• annotate also in the README that pipx install fades is a proper way of using the project (validate first!)
• remove those restrictions that we added to alert the user