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
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).