[build] Fix DFU on recent Python versions and drop Python 2

build/device/dfu.py

@@ -79,7 +79,7 @@
 # Python 3 deprecated getargspec in favour of getfullargspec, but
 # Python 2 doesn't have the latter, so detect which one to use
 import inspect
-getargspec = getattr(inspect, 'getfullargspec', inspect.getargspec)
+getargspec = getattr(inspect, 'getfullargspec', inspect.getfullargspec)
 
 if 'length' in getargspec(usb.util.get_string).args:
     # PyUSB 1.0.0.b1 has the length argument