py3: more robustly cast UUID to bytes
The UUID type only has __str__ implemented. So we need to cast to
bytes on Python 3.
We need an actual bytes instance here (bytestr won't do) because
the re.escape() later iterates over characters and characters
need to behave like ints, not bytes instances of length 1.
Differential Revision: https://phab.mercurial-scm.org/D2144
py3: use bytes() to cast context instances
__str__ and __bytes__ are both implemented on context types. However,
__str__ behaves differently on Python 2 and 3.
Differential Revision: https://phab.mercurial-scm.org/D2143