Support new step api

Since gym==0.25.0, gym warns about the new step api.

/home/mi/mhocke/.venv/jl3/lib/python3.8/site-packages/gym/core.py:329:
DeprecationWarning: WARN: Initializing wrapper in old step API which returns
one bool instead of two. It is recommended to set `new_step_api=True` to use
new step API. This will be the default behaviour in future.

The new api adds a new boolean to the tuple returned by env.step.