context: drop support for changeid of type long (API?)

I don't see a reason to support type long. It's pretty much the same
type as int. There was some discussion about it on the mailing list
around the time of ff2f90503d64 (context: work around `long` not
existing on Python 3, 2017-03-11), but I couldn't find a good reason
to keep it. There was some mention of hgtk doing "repo[long(rev)]",
but that was in 2012.

Differential Revision: https://phab.mercurial-scm.org/D2989

mercurial/context.py

@@ -419,8 +419,6 b' class changectx(basectx):'
                 self._node = repo.changelog.node(changeid)
                 self._rev = changeid
                 return
-            if not pycompat.ispy3 and isinstance(changeid, long):
-                changeid = "%d" % changeid
             if changeid == 'null':
                 self._node = nullid
                 self._rev = nullrev