typing: fix the hint for the `mode` argument of `platform.copymode()`
The posix module is doing a bitwise AND with this and an integer, so it can't be
bytes. The only caller that provides the argument is `util.mktempcopy()`, and
pytype infers the type as Any, which explains why this wasn't caught.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}