👌 IMPROVE: `virtualenv`/`virtualenvwrapper` integration

[mbercx]

The current approach still has some gaps in it (i.e. it doesn't install virtualenv by default), and in practise I still rely on the workon command of virtualenvwrapper without explicitly installing it. We should work on making this as seamless as possible in all possible machine setups.

[mbercx]

Actually, perhaps we shouldn't use virtualenv at all, and simply use the venv module which is part of the standard library since Python 3.3. I don't immediately see an advantage of using virtualenv vs venv (also see this thread), and perhaps integration with virtualenvwrapper is also unnecessary.

[mbercx]

Partially fixed in bf7b619.

Using the standard library package venv is definitely preferable, and the package will still look to see if WORKON_HOME has been set (which is the case for users of virtualenvwrapper). However, we should perhaps still add instructions for users that want to integrate virtualenvwrapper after starting to use aiida-project, i.e. point them to the correct WORKON_HOME setting to integrate this.