python-chess v0.26.0

This is the first **release candidate for python-chess 1.0**. If you see the
need for breaking changes, please speak up now!

Changes:

* `chess.engine` **is now stable and replaces**
  `chess.uci` **and** `chess.xboard`.
* Advanced: `EngineProtocol.initialize()` is now public for use with custom
  transports.
* Removed `__ne__` implementations (not required since Python 3).
* Assorted documentation and coding-style improvements.

New features:

* Check insufficient material for a specific side:
  `board.has_insufficient_material(color)`.
* Copy boards with limited stack depth: `board.copy(stack=depth)`.

Bugfixes:

* Properly handle delayed engine errors, for example unsupported options.