tools/mpy-tool.py: Adjust use of super() to make it work with Python 2.
tools/mpy-tool.py: Adjust use of super() to make it work with Python 2.

Fixes the regression introduced in ea3c80a514c5dc4cc3a8349815eceec4fa1ac57f

adafruit/circuitpython,kerneltask/micropython,bvernoux/micropython,pfalcon/micropython,pramasoul/micropython,MrSurly/micropython,kerneltask/micropython,trezor/micropython,bvernoux/micropython,trezor/micropython,selste/micropython,pramasoul/micropython,henriknelson/micropython,pramasoul/micropython,bvernoux/micropython,henriknelson/micropython,adafruit/circuitpython,MrSurly/micropython,MrSurly/micropython,pfalcon/micropython,selste/micropython,trezor/micropython,pozetroninc/micropython,selste/micropython,pramasoul/micropython,tobbad/micropython,MrSurly/micropython,kerneltask/micropython,pozetroninc/micropython,selste/micropython,henriknelson/micropython,pfalcon/micropython,bvernoux/micropython,selste/micropython,henriknelson/micropython,pozetroninc/micropython,tobbad/micropython,pozetroninc/micropython,adafruit/circuitpython,tobbad/micropython,henriknelson/micropython,pozetroninc/micropython,adafruit/circuitpython,pramasoul/micropython,kerneltask/micropython,adafruit/circuitpython,pfalcon/micropython,bvernoux/micropython,tobbad/micropython,pfalcon/micropython,tobbad/micropython,trezor/micropython,adafruit/circuitpython,trezor/micropython,MrSurly/micropython,kerneltask/micropython