Multiprocessing pickle challenges

[ubershmekel]

Since pillow "Added support for encoding and decoding iTXt chunks"
python-pillow/Pillow#818

Certain images with those chunks are now unpicklable.
python-pillow/Pillow#1434

Causing this failure:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\Users\myuser\AppData\Local\Continuum\Anaconda3\lib\multiprocessing\spawn.py", line 105, in spawn_main
    exitcode = _main(fd)
  File "C:\Users\myuser\AppData\Local\Continuum\Anaconda3\lib\multiprocessing\spawn.py", line 115, in _main
    self = reduction.pickle.load(from_parent)
TypeError: __new__() missing 2 required positional arguments: 'lang' and 'tkey'
Traceback (most recent call last):
  File "C:/Users/myuser/Downloads/mosaic/mosaic.py", line 236, in <module>
    mosaic(sys.argv[1], sys.argv[2])
  File "C:/Users/myuser/Downloads/mosaic/mosaic.py", line 230, in mosaic
    compose(image_data, tiles_data)
  File "C:/Users/myuser/Downloads/mosaic/mosaic.py", line 204, in compose
    Process(target=build_mosaic, args=(result_queue, all_tile_data_large, original_img_large)).start()
  File "C:\Users\myuser\AppData\Local\Continuum\Anaconda3\lib\multiprocessing\process.py", line 105, in start
    self._popen = self._Popen(self)
  File "C:\Users\myuser\AppData\Local\Continuum\Anaconda3\lib\multiprocessing\context.py", line 223, in _Popen
    return _default_context.get_context().Process._Popen(process_obj)
  File "C:\Users\myuser\AppData\Local\Continuum\Anaconda3\lib\multiprocessing\context.py", line 322, in _Popen
    return Popen(process_obj)
  File "C:\Users\myuser\AppData\Local\Continuum\Anaconda3\lib\multiprocessing\popen_spawn_win32.py", line 65, in __init__
    reduction.dump(process_obj, to_child)
  File "C:\Users\myuser\AppData\Local\Continuum\Anaconda3\lib\multiprocessing\reduction.py", line 60, in dump
    ForkingPickler(file, protocol).dump(obj)
BrokenPipeError: [Errno 32] Broken pipe