Pin Gym to 0.19

[avnishn]

Gym appears to have cut a release, 0.21.
It isn't clear what changes were made
between 0.19/0.20 and 0.21, as there is
no change log available for the 0.21 release,
so for now we'll pin gym to 0.19 until we
can fully understand the breaking changes
in gym 0.21. I suspect some things have
just been removed from the regular gym installation
that rllib has previously relied on. Will address
later.

Why are these changes needed?

Related issue number

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[simon-mo]

Please double check the rllib test passing.

[gjoliver]

learning test is still failing with the same erorr message.

I wonder if the real issue is that we are only installing gym[atari] here
https://github.com/ray-project/ray/blob/master/python/requirements/ml/requirements_rllib.txt#L7

maybe with this release, gym[atari] will not install the basic gym envs anymore.
so try adding gym in requirements_rllib.txt?