Open Spiel 1.6.1

This is a staging release in preparation for OpenSpiel 2.0 and in conjunction with the Kaggle Game Area.

A full changelog (from 1.6 -> 2.0) will be made available in the OpenSpiel 2.0 release.

Acknowledgments

Thanks to Google DeepMind for continued support of development and maintenance of OpenSpiel.

Thanks to all of our contributors:

• Core Team: https://github.com/deepmind/open_spiel/blob/master/docs/authors.md
• All Contributors: https://github.com/deepmind/open_spiel/graphs/contributors

Special thanks to the Kaggle Game Arena team and to other internal projects for help in developing ongoing design improvements to the core API.