fix pydfu.py script compat

Author: scgbckbone

tools/pydfu.py

@@ -82,7 +82,7 @@
 import inspect
 # Python 3 deprecated getargspec in favour of getfullargspec, but
 # Python 2 doesn't have the latter, so detect which one to use
-getargspec = getattr(inspect, "getfullargspec", inspect.getargspec)
+getargspec = getattr(inspect, "getfullargspec", getattr(inspect, "getargspec", None))
 
 if "length" in getargspec(usb.util.get_string).args:
     # PyUSB 1.0.0.b1 has the length argument