Tools: sim_vehicle.py fix for Python 3

Tools/autotest/sim_vehicle.py

@@ -830,7 +830,7 @@ parser = CompatOptionParser("sim_vehicle.py",
                "you are simulating, for example, start in the ArduPlane directory to " \
                "simulate ArduPlane")
 
-parser.add_option("-v", "--vehicle", type='choice', default=None, help="vehicle type (%s)" % vehicle_options_string, choices=_options.keys())
+parser.add_option("-v", "--vehicle", type='choice', default=None, help="vehicle type (%s)" % vehicle_options_string, choices=list(_options.keys()))
 parser.add_option("-f", "--frame", type='string', default=None, help="""set vehicle frame type
 
 %s""" % (generate_frame_help()))