Remove "stop=" argument for compatibility with CircuitPython busio.I2C

[jepler]

To reflect the behavior of CircuitPython's busio.I2C, remove the "stop=" argument from the I2C.writeto() method, and always send a "stop" condition on the I2C bus at the end of a write: https://github.com/adafruit/Adafruit_CircuitPython_BitbangIO/blob/master/adafruit_bitbangio.py#L124

See adafruit/Adafruit_CircuitPython_BusDevice#53

[makermelissa]

Fixed via #12.