python-chess v0.21.0

Bugfixes:

* `Board.set_piece_at()` no longer shadows optional `promoted`
  argument from `BaseBoard`.
* Fixed `ThreeCheckBoard.is_irreversible()` and
  `ThreeCheckBoard._transposition_key()`.

Changes:

* `chess.pgn.read_pgn()` **no longer adds the default 7 tag roster.**
  For example `game.headers["White"]` may now raise `KeyError` instead of
  providing the default value `?`. Use `game.headers.get("White", "?")` for
  equivalent behavior.
* Documentation fixes and tweaks by Boštjan Mejak.
* Changed unicode character for empty squares in `Board.unicode()`.

New features:

* XBoard: `white`, `black`, `random`, `nps`, `optim`, `undo`, `remove`. Thanks
  to Manik Charan.