cext: use PyLong symbols
We no longer need to support Python 2. So use the Python 3 symbol
names directly.
Differential Revision: https://phab.mercurial-scm.org/D12228
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}