convert code to python3

[apivovarov]

mxnet-ssd/symbol/symbol_factory.py

Line 33 in 821a1c5

for ratio in xrange(min_ratio, max_ratio + 1, step):

python3 does not have xrange

[apivovarov]

Another issue with python3 is No module named 'common'
can be solved by

export PYTHONPATH=.../mxnet-ssd/symbol