contrib: update Windows environment to Python 3.9.9
Let's keep the environment modern.
Differential Revision: https://phab.mercurial-scm.org/D11775
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}