py3: add __bytes__() for mq.patchheader and make sure __str__ returns str
Before this patch, __str__() function for mq.patchheader class return bytes
which is not str on Python 3. So let's move that logic to __bytes__() and for
__str__() convert the return value of __bytes__() to str.
Differential Revision: https://phab.mercurial-scm.org/D2042
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