Uses deprecated stop=False

[jepler]

Remove the code that deals with the case where not hasattr(self.tca.i2c, "writeto_then_readfrom") -- CircuitPython 5.x and 6.x, as well as Blinka, all have writeto_then_readfrom. https://github.com/adafruit/Adafruit_CircuitPython_TCA9548A/blob/master/adafruit_tca9548a.py#L92

[jepler]

see adafruit/Adafruit_CircuitPython_BusDevice#54

[jepler]

Apparently the stop=False block is just backwards compatibility for 3.x and can be removed.