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
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}