Update EnvSpec to improve backward compatibility

[pseudo-rnd-thoughts]

Description

As noted by DLR-RM/stable-baselines3#1327 (comment), there are significant backward compatibility issues with #292

This PR updates EnvSpec such that it is a "full description of the whole env". Using the EnvSpec in make is now identical to make(str) as before. Critically, the difference with make in v0.27 is that the EnvSpec changes with each wrapper applied, including the default wrappers. As a result, env.unwrapped.spec's EnvSpec has all parameters disabled such that gym.make(env.unwrapped.spec) is equivalent to just the base environment.

The advantage of this implementation is that users can define an environment, env and perfectly replicate each stage

while env is not env.unwrapped:
    recreated_env = gym.make(env.spec)
    assert recreated_env == env  # (this line will fail due to the environment equivalent not having a custom function)
    assert recreated_env.spec == env.spec

    env = env.env

Additionally, this PR updates the testing for make such that we check for both strings and EnvSpec.

[pseudo-rnd-thoughts]

Note, this breaks backward compatibility for TimeLimit as previously users were able to do TimeLimit(env) if the env.spec.max_episode_steps is not None as internally TimeLimit would use this value.
Due to the changes in this PR, we need to remove this functionality and personally, I don't think that it makes much sense before.

[rodrigodelazcano]

The PR looks ready to be merged. I've been testing it with Minari to recreate environments from json environment specs and it works perfectly. Thanks @pseudo-rnd-thoughts !

[RedTachyon]

Two minor comments, but I wasn't able to look at this in proper depth. If it works for "normal" usage (i.e. with the branch installed you can create an env, get the spec, recreate it and do regular stuff without any obvious bugs or redundant warnings), and it works for Minari purposes, then looks good to me as well

[RedTachyon]

Why is this test removed?

[pseudo-rnd-thoughts]

I moved the tests to test_make.py I believe

[RedTachyon]

Minor comment for slight unspagghettification - it might be better to do

if self._cached_spec is not None:
    return self._cached_spec

env_spec = self.env.spec
...

Basically just flip the condition and do an early return to remove one level of indentation from the whole code block.

[pseudo-rnd-thoughts]

Updated