context: work around `long` not existing on Python 3

I can't figure out what this branch is even trying to accomplish, and
it was introduced in ac89a23ca814 which doesn't really shed any
insight into why longs are treated differently from ints.

mercurial/context.py

@@ -33,6 +33,7 b' from . import ('
     obsolete as obsmod,
     patch,
     phases,
+    pycompat,
     repoview,
     revlog,
     scmutil,
@@ -419,7 +420,7 b' class changectx(basectx):'
                 self._node = repo.changelog.node(changeid)
                 self._rev = changeid
                 return
-            if isinstance(changeid, long):
+            if not pycompat.ispy3 and isinstance(changeid, long):
                 changeid = str(changeid)
             if changeid == 'null':
                 self._node = nullid