util: cast memoryview to bytes
Python 3 uses readinto() instead of read() in places. And
taking a slice of the buffer passed to readinto() will produce
a memoryview. _writedata() then gets confused when testing for
`b'\n' in data` because memoryview is an iterable over ints
instead of 1 character bytes.
We work around by casting a memoryview to bytes.
Differential Revision: https://phab.mercurial-scm.org/D5704
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