update debian installation instructions to recommend installing `virtualenv`...

update debian installation instructions to recommend installing `virtualenv` instead of `python3-virtualenv` (#6892)

* change debian package from python3-virtualenv to virtualenv

The virtualenv package is needed for the virtualenv command. The
virtualenv package depends on python3-virtualenv (at least since
debian jessie) so there is no need to specify python3-virtualenv
additionally.

Signed-off-by: Vieno Hakkerinen <vieno@hakkerinen.eu>

* Add changelog

Co-authored-by: Andrew Morgan <andrew@amorgan.xyz>