python-chess v0.12.4

Bugfixes:

* Another en passant related Bugfix for pure Python Gaviota tablebase probing.

New features:

* Added `pgn.GameNode.is_end()`.

Changes:

* Big speedup for `pgn` module. Boards are cached less agressively. Board
  move stacks are copied faster.

* Added tox.ini to specify test suite and flake8 options.