hg-docker: fix Python 3.4 compatibility (for CentOS 7)
I realize Mercurial is not targetting Python 3.4 compatibility,
but without this change, it's not even possible to build it on
CentOS 7 (and I assume the same is true for RHEL 7).
Differential Revision: https://phab.mercurial-scm.org/D5570
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}