python-chess v0.18.0

Bugfixes:

* Fixed `Board.parse_uci()` for crazyhouse drops. Thanks to Ryan Delaney.
* Fixed `AtomicBoard.is_insufficient_material()`.
* Fixed signature of `SuicideBoard.was_into_check()`.
* Explicitly close input and output streams when a `chess.uci.PopenProcess`
  terminates.
* The documentation of `Board.attackers()` was wrongly stating that en passant
  capturable pawns are considered attacked.

Changes:

* `chess.SquareSet` is no longer hashable (since it is mutable).
* Removed functions and constants deprecated in v0.17.0.
* Dropped `gmpy2` and `gmpy` as optional dependencies. They were no longer
  improving performance.
* Various tweaks and optimizations for 5% improvement in PGN parsing and perft
  speed. (Signature of `_is_safe` and `_ep_skewered` changed).
* Rewritten `chess.svg.board()` using `xml.etree`. No longer supports *pre* and
  *post*. Use an XML parser if you need to modify the SVG. Now only inserts
  actually used piece defintions.
* Untangled UCI process and engine instanciation, changing signatures of
  constructors and allowing arbitrary arguments to `subprocess.Popen`.
* Coding style and documentation improvements.

New features:

* `chess.svg.board()` now supports arrows. Thanks to @rheber for implementing
  this feature.
* Let `chess.uci.PopenEngine` consistently handle Ctrl+C across platforms
  and Python versions. `chess.uci.popen_engine()` now supports a `setpgrp`
  keyword argument to start the engine process in a new process group.
  Thanks to @dubiousjim.
* Added `board.king(color)` to find the (royal) king of a given side.
* SVGs now have `viewBox` and `chess.svg.board(size=None)` supports and
  defaults to `None` (i.e. scaling to the size of the container).