contrib: drop python 3.7 from the Windows dependency installer script
I'm tempted to drop 3.8 too, since we use 3.9 on Windows.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}