Bugfixes:
* `CrazyhouseBoard.root()` and `ThreeCheckBoard.root()` were not returning the
correct pockets and number of remaining checks, respectively. Thanks @gbtami.
* `chess.pgn.skip_game()` now correctly skips PGN comments that contain
line-breaks and PGN header tag notation.
Changes:
* Renamed `chess.pgn.GameModelCreator` to `GameCreator`. Alias kept in place
and will be removed in a future release.
* Renamed `chess.engine` to `chess._engine`. Use re-exports from `chess.uci`
or `chess.xboard`.
* Renamed `Board.stack` to `Board._stack`. Do not use this directly.
* Improved memory usage: `Board.legal_moves` and `Board.pseudo_legal_moves`
no longer create reference cycles. PGN visitors can manage headers
themselves.
* Removed previously deprecated items.
Features:
* Added `chess.pgn.BaseVisitor.visit_board()` and `chess.pgn.BoardCreator`.