python-chess v1.6.0

New features:

* Allow offering a draw to XBoard engines using
  ``chess.engine.Protocol.play(..., draw_offered=True)``.
* Now detects insufficient material in Horde. Thanks @stevepapazis!

Changes:

* ``chess.engine.popen_engine(..., setpgrp=True)`` on Windows now merges
  ``CREATE_NEW_PROCESS_GROUP`` into ``creationflags`` instead of overriding.
  On Unix it now uses ``start_new_session`` instead of calling ``setpgrp`` in
  ``preexec_fn``.
* Declare that ``chess.svg`` produces SVG Tiny 1.2, and prepare SVG 2 forwards
  compatibility.

Bugfixes:

* Fix slightly off-center pawns in ``chess.svg``.
* Fix typing error in Python 3.10 (due to added ``int.bit_count``).