Fix HanabiState.copy setting a bad pointer for _game member

[98devin]

Fixes HanabiState.fireworks() returning incorrect data when called on a copied HanabiState object.

Came across this bug in work-related project.
The bug is pretty easily fixed by retrieving the parent game of the state into an existing pyhanabi_game_t* instead of returning a void pointer as was done before.

Steps to reproduce the bug as it existed:

• Create a HanabiState object by calling HanabiState.copy or using the copy constructor manually
• HanabiState._game will receive a hanabi_learning_env::HanabiGame* rather than pyhanabi_game_t*.
• HanabiState.fireworks() and any derived HanabiObservation.fireworks() calls will return bad data, because lib.NumPlayers/lib.NumColors/lib.NumRanks will access garbage memory when called on the resulting _game field.

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[NeilBurch]

Hi 98devin,
Thank you for the suggested fix. It looks right to me. Could you please sign the CLA, and I'll merge those changes?