[python] Drop PY2 support for dm_env module test.
[python] Drop PY2 support for dm_env module test.

The dm_env library itself has dropped PY2 support:

https://github.com/deepmind/dm_env/commit/09686ebd967d8ca5fdb3edf8115f0de3e0912569

deepmind/lab2d,deepmind/lab2d,deepmind/lab2d