python-chess v0.31.3

Bugfixes:

* Custom castling rights assigned to `board.castling_rights` or castling rights
  left over after `Board.set_board_fen()` were not correctly cleaned after
  the first move.

Changes:

* Ignore up to one consecutive empty line between PGN headers.
* Added PGN Variant `From Position` as an alias for standard chess.
* `chess.pgn.FileExporter.result()` now returns the number of written
  characters.
* `chess.engine` now avoids sending 0 for search limits, which some engines
  misunderstand as no limit.
* `chess.engine` better handles null moves sent to the engine.
* `chess.engine` now gracefully handles `NULL` ponder moves and uppercase
  moves received from UCI engines, which is technically invalid.

New features:

* Added `chess.pgn.GameNode.{clock, set_clock}()` to read and write
  `[%clk ...]` **PGN annotations**.
* Added `chess.pgn.GameNode.{arrows, set_arrows}()` to read and write
  `[%csl ...]` and `[%cal ...]` PGN annotations.
* Added `chess.pgn.GameNode.{eval, set_eval}()` to read and write
  `[%eval ...]` PGN annotations.
* Added `SquareSet.ray(a, b)` and `SquareSet.between(a, b)`.