ci: remove py2-rust support
Nobody cares about this very narrow usecase, and py2 support is over by
July 1st. This helps with the CI load, and removes some flakiness.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}