mujoco_py_env: Only raise an exception if MuJocoPyEnv class is actually used

[traversaro]

Description

Before #934 just running from gymnasium.envs.mujoco import MuJocoPyEnv did not resulted in a error.DependencyNotInstalled exception, but the expection was only raised when MuJocoPyEnv.__init__ constructor was called.

Instead after #934 just calling from gymnasium.envs.mujoco.mujoco_py_env import MuJocoPyEnv results in a error.DependencyNotInstalled exception, that I guess create an hard error during test collection.

This result in an hard error when collecting tests in conda-forge/gymnasium-feedstock#36, where mujoco_py is not available. Note that there is a similar change in #934 related to mujoco module itself, but that is less problematic as it a maintained library, differently from mujoco_py.

Type of change

Please delete options that are not relevant.

• Documentation only change (no code changed)
• 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

Not relevant.

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

[traversaro]

Sorry, I did the modifications online and I did not run pre-commit, I will fix it.

[traversaro]

Sorry, I did the modifications online and I did not run pre-commit, I will fix it.

Fixed.