labelit.png broken in python 3

[Anthchirp]

in a python 3 environment run eg.

$ labelit.png dials_regression/image_examples/DESY_ID141/q210_2_001.img
Traceback (most recent call last):
  File "cctbx_project/iotbx/command_line/detector_image_as_png.py", line 76, in <module>
    run(sys.argv[1:])
  File "cctbx_project/iotbx/command_line/detector_image_as_png.py", line 33, in run
    C = convert_image(img_file,graphics_bin)
  File "cctbx_project/iotbx/command_line/detector_image_as_png.py", line 60, in __init__
    data_string = flex_img.export_string
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xff in position 0: invalid start byte