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
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