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

tests: look for ensurepip before using venv Debian appears to cripple the venv module by default by removing the associated ensurepip functionality. (The module isn't present at all.) This caused test-install.t to fail when using the Debian python3 unless the python3-venv package was installed. This commit introduces a new hghave requirement for detecting ensurepip and makes the Python 3 install variant conditional on its presence. This should make test-install.t pass when using an incomplete Debian Python. Differential Revision: https://phab.mercurial-scm.org/D7228

automation: avoid '~' in the temp directory on Windows If a long-ish username is used, the environment variable ends up with a '~' to be 8.3 path compatible. That in turn causes a handful of tests (mostly ssh related) to add quotes around $TESTMP. I have no AWS experience, so I have no idea if this is the proper way to do it. But I've hit this problem locally, and redirecting the directory is a workaround. I don't recall if the directory is created on demand by the test harness, but presumably if this is configured before the machine boots, Windows will do it for us. Differential Revision: https://phab.mercurial-scm.org/D7130