virtualenv is a tool to create isolated Python environments, each invokable
 with its own Python executable. Each instance can have different sets of
 modules, installable via pip. Virtual Python instances can also be created
 without root access.
 .
 Since Python 3.3, a subset of it has been integrated into the standard library
 under the venv module (python3-venv in Debian). The venv module does not
 offer all features of this library, to name just a few more prominent ones:
 .
  * is slower (by not having the app-data seed method),
  * is not as extendable,
  * cannot create virtual environments for arbitrarily installed Python
    versions (and automatically discover these),
  * does not have as rich programmatic API (describe virtual environments
    without creating them).
 .
 This is the Python 3 version of the library. It includes the command line
 script.