python-chess v0.20.0

Bugfixes:

* Some XBoard commands were not returning futures.
* Support semicolon comments in PGNs.

Changes:

* Changed FEN and EPD formatting options. It is now possible to include en
  passant squares in FEN and X-FEN style, or to include only strictly relevant
  en passant squares.
* Relax en passant square validation in `Board.set_fen()`.
* Ensure `is_en_passant()`, `is_capture()`, `is_zeroing()` and
  `is_irreversible()` strictly return bools.
* Accept `Z0` as a null move in PGNs.

New features:

* XBoard: Add `memory`, `core`, `stop` and `movenow` commands.
  Abstract `post`/`nopost`. Initial `FeatureMap` support. Support `usermove`.
* Added `Board.has_pseudo_legal_en_passant()`.
* Added `Board.piece_map()`.
* Added `SquareSet.carry_rippler()`.
* Factored out some (unstable) low level APIs: `BB_CORNERS`,
  `_carry_rippler()`, `_edges()`.