fixes #4 caused by CP's liberal unpack

[siddacious]

This should fix the issue; As far as I can tell CP's unpack is filling in the missing byte which we are already shifting away, so increasing _BUFFER to 4 bytes allows CPython's unpack to not barf and the garbage extra byte at the end isn't a problem.

I tested that it works in Python 3 by pasting and example _BUFFER into a REPL and finishing the conversion by hand, but if someone can double check on an RPi or similar that would be swell.

~$ python3
Python 3.7.0 (default, Sep 25 2018, 01:24:12)
[Clang 7.0.2 (clang-700.1.81)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> raw = bytearray(b'\x01P ')
>>> from struct import unpack
>>> unpack(">i", raw)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
struct.error: unpack requires a buffer of 4 bytes
>>> unpack(">i", bytearray(b'\x01I\xa0\x00'))
(21602304,)
>>> len(bytearray(b'\x01I\xa0\x00'))
4
>>> unpack(">i", bytearray(b'\x01I\xa0\x00'))
(21602304,)
>>> unpack(">i", bytearray(b'\x01I\xa0\x00'))[0]>>8
84384
>>> (unpack(">i", bytearray(b'\x01I\xa0\x00'))[0]>>8)/4096.0
20.6015625

An example of CP's unpack behavior:

>>> unpack(">i", bytearray([1,0,0,0]))
(16777216,)
>>> unpack(">i", bytearray([1,0,0]))
(16777216,)
>>> unpack(">i", bytearray([1,0]))
(16777216,)
>>> unpack(">i", bytearray([1]))
(16777216,)

[caternuson]

Elegant. Tested and works on RPi.

pi@raspberrypi:~ $ python3
Python 3.5.3 (default, Sep 27 2018, 17:25:39) 
[GCC 6.3.0 20170516] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import board, busio
>>> import digitalio
>>> import adafruit_max31856
>>> spi = busio.SPI(board.SCK, board.MOSI, board.MISO)
>>> cs = digitalio.DigitalInOut(board.D21)
>>> cs.direction = digitalio.Direction.OUTPUT
>>> tc = adafruit_max31856.MAX31856(spi, cs)
>>> len(tc._BUFFER)
4
>>> tc.temperature
26.1171875
>>> 

@ladyada thanks for the tip about setting PYTHONPATH for testing CP libs on RPi/blinka

[caternuson]

@siddacious Did / can you test this on a CP board?

[siddacious]

@caternuson Yes, I did.

[caternuson]

Looks good to me then. Thanks.

[ladyada]

alright @caternuson @kattni bump and release :)

[kattni]

Done!