This change fixes the 6 unit tests in

https://github.com/deepmind/open_spiel/tree/bbfb0259b5d26c1e3f05000f1f4a05b7ad60393e/open_spiel/python/mfg/algorithms
    best_response_value_test
    distribution_test
    fictitious_play_test
    greedy_policy_test
    nash_conv_test
    policy_value_test

CAVEAT: The fix is NOT covered by pybind11 unit tests.