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New features:
* Added ``chess.Board.ply()``.
* Added ``chess.pgn.GameNode.ply()`` and ``chess.pgn.GameNode.turn()``.
* Added ``chess.engine.PovWdl``, ``chess.engine.Wdl``, and conversions from
scores: ``chess.engine.PovScore.wdl()``, ``chess.engine.Score.wdl()``.
* Added ``chess.engine.Score.score(*, mate_score: int) -> int`` overload.
Changes:
* The ``PovScore`` returned by ``chess.pgn.GameNode.eval()`` is now always
relative to the side to move. The ambiguity around ``[%eval 0]`` has been
resolved to ``Mate(-0)``. This makes sense, given that the authors of the
specification probably had standard chess in mind (where a game-ending move
is always a loss for the opponent). Previously, this would be parsed as
``None``.
* Typed ``chess.engine.InfoDict["wdl"]`` as the new ``chess.engine.PovWdl``,
rather than ``Tuple[int, int, int]``. The new type is backwards compatible,
but it is recommended to use its documented fields and methods instead.
* Removed ``chess.engine.PovScore.__str__()``. String representation falls back
to ``__repr__``.
* The ``en_passant`` parameter of ``chess.Board.fen()`` and
``chess.Board.epd()`` is now typed as ``Literal["legal", "fen", "xfen"]``
rather than ``str``.