python-chess v0.21.1

Bugfixes:

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

New features:

* Added `Game.without_tag_roster()`. `chess.pgn.StringExporter()` can now
  handle games without any headers.
* XBoard: `white`, `black`, `random`, `nps`, `optim`, `undo`, `remove`. Thanks
  to Manik Charan.

Changes:

* Documentation fixes and tweaks by Boštjan Mejak.
* Changed unicode character for empty squares in `Board.unicode()`.