Update CircuitPython i2c guide

[tannewt]

https://learn.adafruit.com/circuitpython-basics-i2c-and-spi/i2c-devices

Read over it, test examples and update as needed for CircuitPython 3.0.0.

[kattni]

Updated, tested. Changed a single read_into to readinto. Everything else still works as-is.