fix pydfu.py script compat

[scgbckbone]

Traceback (most recent call last):
  File "/home/scg/PycharmProjects/firmware/stm32/../external/micropython/tools/pydfu.py", line 85, in <module>
    getargspec = getattr(inspect, "getfullargspec", inspect.getargspec)
                                                    ^^^^^^^^^^^^^^^^^^
AttributeError: module 'inspect' has no attribute 'getargspec'. Did you mean: 'getargs'?