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

tests: use venv on Python 3 This test was failing in some Python 3 environments because `$PYTHON -m virtualenv` was somehow resulting in Python 2 being used. Why, I'm not sure. Python 3 includes virtualenv in the standard library as the `venv` module. This commit changes test-install.t to use `$PYTHON -m venv` on Python 3 and `$PYTHON -m virtualenv` on Python 2 (if available). I chose to make some test output duplicated because we can't have nested conditionals and there is no easy way to express ORing of hghave checks. Differential Revision: https://phab.mercurial-scm.org/D7224

tests: remove HGALLOWPYTHON3 reference This variable was removed from setup.py in c3e10f705a6c. Differential Revision: https://phab.mercurial-scm.org/D7223

run-tests: use byte strings for inserted output We were inserting str on Python 3 which resulted in mixed str/bytes types on the list. This would later blow up when trying to write str to the .err file opened in bytes mode. Differential Revision: https://phab.mercurial-scm.org/D7222

contrib: require Python 3.7 for byteify-strings.py bb509f39d387 made an error, it's actually 3.7 that introduced token.COMMENT. Differential Revision: https://phab.mercurial-scm.org/D7220

hghave: fix bytes/string issue on Python 3

packaging: add support for CentOS 8 The resulting executable has not been tested in detail yet. I ran 'hg version' and 'hg clone', which worked fine (except for extensions acting up due to Python 3). Differential Revision: https://phab.mercurial-scm.org/D7216

packaging: allow choosing python version depending on centos version Differential Revision: https://phab.mercurial-scm.org/D7217