Switch to Python 2 for compatibility
Much improved MSD radix sort

I tried a naive recursive implementation at first, but it quickly hit
Python's recursion limit, so I fixed it into a stack-based iterative
one.  The sort performed terribly with certains inputs, so I had it
switch to the built-in `sorted` method for small buckets... though
I may have exaggerated what "small" is.  I just picked the best
result empirical testing gave me.