New features:
* Use ``chess.engine.Opponent`` to send opponent information to engines.
* Inform engines about the game result using
``chess.engine.Protocol.send_game_result()``.
* Add ``chess.engine.Limit.clock_id``.
* Add ``chess.svg.board(..., borders=True)``.
* Avoid rendering background behind SVG boards to better support transparency.
* Add ``chess.pgn.BaseVisitor.begin_parse_san()``.
* Introduce new distance metrics ``chess.square_manhattan_distance()`` and
``chess.square_knight_distance()``.
Bugfixes:
* Fix ``chess.pgn.GameNode.eval()`` sometimes off by one centipawn.
* Fix handling of additional spaces between UCI option tokens.
* Handle implicit XBoard engine resignation via output like
``0-1 {White resigns}``.
Changes:
* Add model ``sf16`` for ``chess.engine.Score.wdl()``, the new default.
* Update ``lichess`` WDL model.
* Keep PGN headers that do not belong to the Seven Tag Roster in insertion
order.
* Halve the number of open file descriptors maintained by tablebases
and opening books.
* Reduce verbosity of logged ``chess.pgn`` errors.
