Changes:
* Missing *SetUp* tags in PGNs are ignored.
* Incompatible comparisons on `chess.Piece`, `chess.Move`, `chess.Board`
and `chess.SquareSet` now return *NotImplemented* instead of *False*.
Minor new features:
* Factored out basic board operations to `chess.BaseBoard`. This is inherited
by `chess.Board` and extended with the usual move generation features.
* Added optional *claim_draw* argument to `chess.Base.is_game_over()`.
* Added `chess.Board.result(claim_draw=False)`.
* Allow `chess.Board.set_piece_at(square, None)`.
* Added `chess.SquareSet.from_square(square)`.
