Fix pydfu.py to work with Beta 1 and Beta 2 versions of PyUSB
Fix pydfu.py to work with Beta 1 and Beta 2 versions of PyUSB

In PyUSB 1.0.0.b1, the usb.util.get_string function takes a lenght argument.
In PyUSB 1.0.0.b2, it no longer takes a length argument.

Since python2 seems to come with PyUSB 1.0.0.b1 and python3 seems to come
with 1.0.0.b2, this change allows the same source to work with either
version of PyUSB.

kwagyeman/openmv,iabdalkader/openmv,tianzhihen/openmv,openmv/openmv,SmartArduino/openmv,openmv/openmv,SmartArduino/openmv,iabdalkader/openmv,kwagyeman/openmv,SmartArduino/openmv,iabdalkader/openmv,kwagyeman/openmv,kwagyeman/openmv,openmv/openmv,tianzhihen/openmv,tianzhihen/openmv,SmartArduino/openmv,openmv/openmv,iabdalkader/openmv,tianzhihen/openmv