automation: install python3-venv Debian package

Debian's python install has a crippled venv by default, as it is
lacking ensurepip. When you try to run `python3 -m venv` it tells
you to install `python3-venv`. So this commit does that in our
automation environment so we can fully test installing Mercurial
using venv+pip with the system Python.

Differential Revision: https://phab.mercurial-scm.org/D7229

contrib/automation/hgautomation/linux.py

@@ -266,6 +266,7 b' PACKAGES="\\'
     python3-fuzzywuzzy \
     python3-pygments \
     python3-vcr \
+    python3-venv \
     rsync \
     sqlite3 \
     subversion \