python-chess v0.30.1

Changes:

* Positions with more than two checkers are considered invalid and
  `board.status()` returns `chess.STATUS_TOO_MANY_CHECKERS`.
* Pawns drops in Crazyhouse are considered zeroing and reset
  `board.halfmove_clock` when played.
* Now validating file sizes when opening Syzygy tables and Polyglot opening
  books.
* Explicitly warn about untrusted tablebase files and chess engines.

Bugfixes:

* Fix Racing Kings game end detection: Black cannot catch up if their own
  pieces block the goal. White would win on the turn, so this did not
  impact the game theoretical outcome of the game.
* Fix bugs discovered by fuzzing the EPD parser: Fixed serialization of
  empty strings, reparsability of empty move lists, handling of non-finite
  floats, and handling of whitespace in opcodes.

Features:

* Added `board.checkers()`, returning a set of squares with the pieces giving
  check.