Alternative fix for gym 0.21 changes.

[heiner]

This is an alternative to #269 to fix #272.

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The most recent version of gym wraps environments with the OrderEnforcing wrapper (https://github.com/openai/gym/blob/master/docs/wrappers.md) if not disabled. This breaks our tests, play.py and a number of other places. For instance, env.seed() with more than 1 argument now also fails due to gym.Wrapper.seed. Possible fixes:

• Use env.unwrapped when arguments to seed are given
• Disable the OrderEnforcing wrapper. This will break NLE when using earlier versions of the gym library unless we do per-version magic (as in this PR). We cannot test for the presence of OrderEnforcing as it's not exported into gym.wrappers (sigh).
• Adhere to the gym environment better by doing seed(self, (core, disp, reseed)) instead of seed(self, core=None, disp=None, reseed=False).

We also use env._actions a bunch of times, in tests and in play.py. We should fix that, and other accesses to _underscored variables.

[cdmatters]

I prefer this fix over #269 since the change is localised, and doesnt change behaviour for end users who chose to build on this. We should fix this properly later.