githelp: fix a `str` type conditional for py3

Differential Revision: https://phab.mercurial-scm.org/D7537

hgext/githelp.py

@@ -115,7 +115,7 b' def parseoptions(ui, cmdoptions, args):'
     args = list([convert(x) for x in args])
     opts = dict(
         [
-            (k, convert(v)) if isinstance(v, str) else (k, v)
+            (k, convert(v)) if isinstance(v, bytes) else (k, v)
             for k, v in pycompat.iteritems(opts)
         ]
     )