There is an Adafruit_I2C.py module at the root of the source tree, which seems to be several years old. A comment inside directs developers to keep it in sync with https://github.com/adafruit/Adafruit-Raspberry-Pi-Python-Code/tree/master/Adafruit_I2C (which is now https://github.com/adafruit/Adafruit_Python_GPIO/blob/master/Adafruit_GPIO/I2C.py), which has much more recent changes.

As far I can tell, Adafruit_I2C.py here is obsolete and could be removed. Alternatively, it would need to be brought up to date and possibly moved to the Adafruit_BBIO folder for consistency with the other pure Python modules.