py3: replace file() with open() in transplant.py

Differential Revision: https://phab.mercurial-scm.org/D2267

hgext/transplant.py

@@ -274,7 +274,7 b' class transplanter(object):'
                                     },
                            onerr=error.Abort, errprefix=_('filter failed'),
                            blockedtag='transplant_filter')
-            user, date, msg = self.parselog(file(headerfile))[1:4]
+            user, date, msg = self.parselog(open(headerfile, 'rb'))[1:4]
         finally:
             os.unlink(headerfile)