msgpack.unpack requires a readable stream

[prplz]

msgpack.unpack should require read, not write.
This crashes circuitpy if a write-only stream is passed to unpack:

import msgpack
import usb_midi
 # get a write only stream
_, port_out = usb_midi.ports
# before fix: faults
# after fix: OSError: stream operation not supported
msgpack.unpack(port_out)

[dhalbert]

Makes sense. Thanks!