Remove PIL dependency in Sudoku Example

[krachbumm3nte]

As discussed in #2441, I have updated the visualization backend for the Sudoku example to rely fully on matplotlib and imageio.

[hakonsbm]

Thanks! In addition to my suggestions below, the sudoku_solver.py script also needs to be updated to work with the changed function arguments in helpers.py.

[hakonsbm]

Thanks for the changes. I have a few more suggestions.

[hakonsbm]

Thanks!

[heplesser]

@krachbumm3nte Could you pull master into this branch? Then the macOS test should pass (see #2515).

[jougs]

Many thanks for this PR. Other than a couple of formatting issues, I'm fine.

[jougs]

Thanks for the quick reaction. I think this looks much better now.

[heplesser]

@krachbumm3nte Thanks a lot, I will merge this now. I noticed that many docstrings begin with lowercase letters, e.g.,

def get_puzzle(puzzle_index):
     """returns one of 8 Sudoku configuration to be solved.

Our style is to begin docstrings with capital letters, e.g., here """Returns one ...". Could you at some point create another PR fixing this?