python-chess v1.5.0

Bugfixes:

* Fixed typing of ``chess.pgn.Mainline.__reversed__()``. It is now a generator,
  and ``chess.pgn.ReverseMainline`` has been **removed**.
  This is a breaking change but a required bugfix.
* Implement UCI **ponderhit** for consecutive calls to
  ``chess.engine.Protocol.play(..., ponder=True)``. Previously, the pondering
  search was always stopped and restarted.
* Provide the full move stack, not just the position, for UCI pondering.
* Fixed XBoard level in sudden death games.
* Ignore trailing space after ponder move sent by UCI engine.
  Previously, such a move would be rejected.
* Prevent cancelling engine commands after they have already been cancelled or
  completed. Some internals (``chess.engine.BaseCommand``) have been changed to
  accomplish this.

New features:

* Added ``chess.Board.outcome()``.
* Implement and accept usermove feature for XBoard engines.

Special thanks to @MarkZH for many of the engine related changes in this
release!