Fix mujoco_py only installation

[YouJiacheng]

Only import mujoco_rendering when mujoco installed. Otherwise the try import code in gym.envs.mujoco.mujoco_env is useless.

Description

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Fixes # (issue)

Type of change

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• 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

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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 commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• 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

[YouJiacheng]

It is tricky that the test expect an ImportError. But if there is an ImportError, then

if MUJOCO_NOT_INSTALLED:
    raise error.DependencyNotInstalled(
        f"{MUJOCO_IMPORT_ERROR}. (HINT: you need to install mujoco)"
    )

will be useless.
I'd like to split the file, and raise error.DependencyNotInstalled in the file. Then also catch DependencyNotInstalled in the tests.

Update: Actually we don't need raise this error during import because make will trigger __init__, current implementation already raise this error in __init__. Just catch DependencyNotInstalled in the test is enough!

[pseudo-rnd-thoughts]

What test do you mean? If you can solve a way that mujoco_py works without mujoco and mujoco works without mujoco_py then could you do that

[YouJiacheng]

all test import test.env.utils

gym/tests/envs/utils.py

Lines 11 to 31 in 824507b

	def try_make_env(env_spec: EnvSpec) -> Optional[gym.Env]:
	"""Tries to make the environment showing if it is possible.
	Warning the environments have no wrappers, including time limit and order enforcing.
	"""
	# To avoid issues with registered environments during testing, we check that the spec entry points are from gym.envs.
	if "gym.envs." in env_spec.entry_point:
	try:
	return env_spec.make(disable_env_checker=True).unwrapped
	except ImportError as e:
	logger.warn(f"Not testing {env_spec.id} due to error: {e}")
	return None
	# Tries to make all environment to test with
	all_testing_initialised_envs: List[Optional[gym.Env]] = [
	try_make_env(env_spec) for env_spec in gym.envs.registry.values()
	]
	all_testing_initialised_envs: List[gym.Env] = [
	env for env in all_testing_initialised_envs if env is not None
	]

[pseudo-rnd-thoughts]

Oh, could you just add a DeprecatedError to the try except. That is expected to fail for python 3.6

[pseudo-rnd-thoughts]

@YouJiacheng could you fix the PR?

[YouJiacheng]

I have found a way to fix it: Catch both ImportError and error.DependencyNotInstalled in test.env.utils.
But I also want to remove from gym.envs.mujoco.mujoco_rendering import RenderContextOffscreen, Viewer from __init__.py to simplify the code!

[pseudo-rnd-thoughts]

For the catching both errors, see https://stackoverflow.com/questions/6470428/catch-multiple-exceptions-in-one-line-except-block

If it is necessary to remove that, then it will be a necessary breaking change

[pseudo-rnd-thoughts]

@YouJiacheng This looks good, could you fix the pre-commit issues

[YouJiacheng]

• Thanks, I know how to catch both error in one line.
• Not really necessary.
• My PC have some problems with pre-commit environment install, I'm trying to fix it! (Finally I run black manually...)

pre-commit/pre-commit#2402
https://superuser.com/questions/1493887/what-is-the-default-registry-value-for-command-processors-autorun