set pickle protocol to 2 for py 2/3 compatibility of pickle files. as per https://stackoverflow.com/questions/25843698/valueerror-unsupported-pickle-protocol-3-python2-pickle-can-not-load-the-file and https://docs.python.org/2/library/pickle.html#data-stream-format
set pickle protocol to 2 for py 2/3 compatibility of pickle files.
as per https://stackoverflow.com/questions/25843698/valueerror-unsupported-pickle-protocol-3-python2-pickle-can-not-load-the-file
and https://docs.python.org/2/library/pickle.html#data-stream-format