py3: use util.forcebytestr to convert testedwith value to bytes

Bad extensions can put anything in testedwith so we should use util.forcebytestr
here.

Differential Revision: https://phab.mercurial-scm.org/D2663

mercurial/dispatch.py

@@ -990,7 +990,7 b' def _exceptionwarning(ui):'
     if worst[0] is not None:
         name, testedwith, report = worst
         if not isinstance(testedwith, (bytes, str)):
-            testedwith = '.'.join([str(c) for c in testedwith])
+            testedwith = '.'.join([util.forcebytestr(c) for c in testedwith])
         warning = (_('** Unknown exception encountered with '
                      'possibly-broken third-party extension %s\n'
                      '** which supports versions %s of Mercurial.\n'