Fix up for Python 3.x

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Two issues needed fixed. First spidev is not buffered so we need to open it as an unbuffered binary file. Second ctypes.string_at returns a bytes object instead of a str in python 3.x so we don't want to convert a str to an array of integers when returning from read and transfer, since we already have arrays of ints.

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Should probably bump version so PyPi and pip see this as an update. I didn't because it could be a minor or a micro depending on opinion.