py3: use pycompat.byteskwargs() to covert keys of kwargs to bytes

Differential Revision: https://phab.mercurial-scm.org/D5286

hgext/fastannotate/context.py

@@ -156,6 +156,7 b' class annotateopts(object):'
     }
 
     def __init__(self, **opts):
+        opts = pycompat.byteskwargs(opts)
         for k, v in self.defaults.iteritems():
             setattr(self, k, opts.get(k, v))