sprites.py imports from codebug_tether

sprites.py in codebug_i2c_tether imports from codebug_tether instead of from codebug_i2c_tether.
While this works, if you have also installed codebug_tether, it is not very intuitive.
The sprites.py file seems to be identical in both tether versions, which means it fails to import when the codebug_tether has not also been installed.