py3: pass system string to socket.getservbyname

Differential Revision: https://phab.mercurial-scm.org/D2139

mercurial/util.py

@@ -2717,7 +2717,7 b' def getport(port):'
         pass
 
     try:
-        return socket.getservbyname(port)
+        return socket.getservbyname(pycompat.sysstr(port))
     except socket.error:
         raise Abort(_("no port number associated with service '%s'") % port)