py3: use util.forcebytestr() to convert push lock error to bytes

mercurial/exchange.py

@@ -522,7 +522,8 b' def push(repo, remote, force=False, revs'
         # source repo cannot be locked.
         # We do not abort the push, but just disable the local phase
         # synchronisation.
-        msg = 'cannot lock source repository: %s\n' % err
+        msg = ('cannot lock source repository: %s\n'
+               % stringutil.forcebytestr(err))
         pushop.ui.debug(msg)
 
     with wlock or util.nullcontextmanager(), \