automation: drop support for Python 2.7 in Linux environment
We stop installing Python 2.7 via pyenv. We stop installing the system
Python 2 packages. We delete support for running tests on Python 2.7.
Differential Revision: https://phab.mercurial-scm.org/D12262
automation: make system3 the default for run-tests-linux
We'll soon drop support for Python 2.7. Let's use Python 3 by
default.
Differential Revision: https://phab.mercurial-scm.org/D12261