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>

INSTALL.md

@@ -112,7 +112,7 @@ Installing prerequisites on Ubuntu or Debian:
 ```
 sudo apt-get install build-essential python3-dev libffi-dev \
                      python3-pip python3-setuptools sqlite3 \
-                     libssl-dev python3-virtualenv libjpeg-dev libxslt1-dev
+                     libssl-dev virtualenv libjpeg-dev libxslt1-dev
 ```
 
 #### ArchLinux

changelog.d/6892.doc

@@ -0,0 +1 @@
+Update Debian installation instructions to recommend installing the `virtualenv` package instead of `python3-virtualenv`.