Bugfixes:
* Backport Bugfix for Syzygy DTZ related to en-passant.
See official-stockfish/Stockfish@6e2ca97d93812b2.
Changes:
* Added optional argument *max_fds=128* to `chess.syzygy.open_tablebases()`.
An LRU cache is used to keep at most *max_fds* files open. This allows using
many tables without running out of file descriptors.
Previously all tables were opened at once.
* Syzygy and Gaviota now store absolute tablebase paths, in case you change
the working directory of the process.
* The default implementation of `chess.uci.InfoHandler.score()` will no longer
store score bounds in `info["score"]`, only real scores.
* Added `Board.set_chess960_pos()`.
* Documentation improvements.
