pystockfish.py updated to work well with Python3

[mbadawi23]

This pull request is mainly to upgrade pystockfish.py to work well with Python3. Here is a summary of the main modifications I made:

• Added self.stdin.flush() to Engine.put(), otherwise Engine.put() hangs in Python3.
• All triple single quotes (''') are now replaced with triple double-quotes (''''). Just to match the docstring convention.
• Replaced all tab indentation with space indentation. Again, this is just to match the convention.
• Engine.isready() now returns 'readyok' instead of ''. I think this is more useful.

[iamjarret]

I'm attentive to the care of your edits. Many thanks, mbadawi23, for your contribution.