Add pickling tests, adapt all envs to be picklable

[elliottower]

Description

This PR adds tests to check that each environment can be saved and loaded via pickling. Waterworld and mpe simple envs both have pygame objects (pygame.Clock and pygame Surface, respectively) and cannot be pickled without EzPickle. I have removed the changes I made to version numbers and other miscellaneous changes to make this PR as simple as possible.

Previous comments:

and includes them in the API test. I'm setting this as a draft because I'm not sure if we should include it in the API test, as that includes third-party environments which may not care about pickling. But perhaps it is good to enforce that all environments can be pickled, even custom ones? All tests pass internally.

I have moved them to a separate file from API tests as it was messy and causing other tests to fail due to rendering and video issues, also seems simplest to do it this way.

Fixes # (issue), Depends on # (pull request)

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Screenshots

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Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• I have run pytest -v and no errors are present.
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I solved any possible warnings that pytest -v has generated that are related to my code to the best of my knowledge.
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[elliottower]

Do we care if rendered pygame envs can be pickled? Currently they all work with render_mode=None but with pygame rendering there are some issues like chess has a pygame.time.Clock object which can't be pickled. Could add EzPickle to that file but it's a bit weird to do that as it's all python and I don't imagine people will try to pickle rendered envs. When you load an env which has been pickled and uses pygame it also doesn't have the video system initialized, as the pygame.init() call is in the constructor rather than in reset().
@pseudo-rnd-thoughts @jkterry1

[pseudo-rnd-thoughts]

Repeat comments for the second class

[pseudo-rnd-thoughts]

With the changes, looks good