python-chess v0.18.1

Bugfixes:

* Crazyhouse drops were accepted as pseudo legal (and legal) even if the
  respective piece was not in the pocket.
* `CrazyhouseBoard.pop()` was failing to undo en passant moves.
* `CrazyhouseBoard.pop()` was always returning `None`.
* `Move.__copy__()` was failing to copy Crazyhouse drops.
* Fix ~ order (marker for promoted pieces) in FENs.
* Promoted pieces in Crazyhouse were not communicated with UCI engines.

Changes:

* `ThreeCheckBoard.uci_variant` changed from `threecheck` to `3check`.