commands: use byteskwargs() in verify()

Otherwise Python 3 complains about the missing key.

Differential Revision: https://phab.mercurial-scm.org/D6294

mercurial/commands.py

@@ -6167,6 +6167,8 b' def verify(ui, repo, **opts):'
 
     Returns 0 on success, 1 if errors are encountered.
     """
+    opts = pycompat.byteskwargs(opts)
+
     level = None
     if opts['full']:
         level = verifymod.VERIFY_FULL