py3: fix keyword arguments handling in mq
This patch fixes the handling of keyword arguments to functions on Python 3. On
python3, the keys of keyword arguments need to str which is unicode. So any
keyword argument will get will have str keys and any dictionary we pass as
kwargs must have all the keys as str.
This patch uses pycompat.(strkwargs|byteskwargs) to do so conversion between
bytes keys and str keys and use r'' if there are very less uses and conversion
can be prevented.
Differential Revision: https://phab.mercurial-scm.org/D972
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