End of JOSS review: player copying, game metaclass and docs fixes.

• Players are now copied via copy.deepcopy when a game is created and the
  copies are used, leaving any originals unchanged.
• Formalise test and Python requirements in setup.py.
• Revert flaky forgetting fix from v1.2.1 and correct the flaky tests that were
  causing the issue.
• Replace the Game class with BaseGame and make it a metaclass via
  abc.ABCMeta.
• Fix bugs in documentation stopping build.