typing: add a trivial type hint to `mercurial/vfs.py`
Since hg
3dbc7b1ecaba, pytype stopped seeing the return value of `rmtree` as
`None`, and substituted `Any`.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}