fix AOJ language resolver to support Python2/3

Even though the common language resolver returns "python2" or "python3", the AOJ language resolver can handle only "python". So, submitting to AOJ in Python is impossible :disappointed: 

I fixed this issue by modifying the AOJ language resolver. Now, we can submit Python code to AOJ as follows:
`git contest submit aoj -p 10000 -s p10000.py -l python2`, or `git contest submit aoj -p 10000 -s p10000.py -l python3` :santa: