Aryan

Make sudoku.py Python3 compatible

1

The pull request fixes the print statements to support python3 and concatenation of a list to a `dict_keys` object [here](https://github.com/akumm2k/pysmt/blob/a1214157332a06404e506feb1c16bf5e3ba274f8/examples/sudoku/sudoku.py#L159).

I landed on this repo via Peter Norvig's [webpage](https://norvig.com/sudoku.html). Some of the terminology used on his page is different than in your implementation. The comments make it easier to understand...