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

--- a/contrib/automation/hgautomation/linux.py	Mon Nov 04 23:42:18 2019 -0800
+++ b/contrib/automation/hgautomation/linux.py	Mon Nov 04 23:44:10 2019 -0800
@@ -266,6 +266,7 @@
     python3-fuzzywuzzy \
     python3-pygments \
     python3-vcr \
+    python3-venv \
     rsync \
     sqlite3 \
     subversion \