The keys of keyword arguments must be str on Python 3 which is unicode. This
patch make sure we pass keyword arguments with keys are str everywhere in this
file and access the keys depending on whether they are bytes or str.
This patch uses pycompat.{byteskwargs|strkwargs} and somewhere it also added r''
to prevent transformer from adding a b'' over there.
opts is modified here, but stropts isn't. Perhaps we shouldn't
keep the copy for long.