docs: pin dependency

[Gaiejj]

Description

Pin the version of sphinx as 7.1.2.

Motivation and Context

Some issue exists in the 7.2.3 version of sphinx.
Read the docs would fail building the online documentation.
The error information is:

  File "/home/docs/checkouts/readthedocs.org/user_builds/omnisafe/envs/latest/lib/python3.9/site-packages/torch/__init__.py", line 1157, in <module>
    from torch._C._VariableFunctions import *  # type: ignore[misc] # noqa: F403
ModuleNotFoundError: No module named 'torch._C._VariableFunctions'; 'torch._C' is not a package

So this pull request pin the version of sphinx in 7.1.2 to solve it.

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds core functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (update in the documentation)

Checklist

Go over all the following points, and put an x in all the boxes that apply.
If you are unsure about any of these, don't hesitate to ask. We are here to help!

• I have read the CONTRIBUTION guide. (required)
• My change requires a change to the documentation.
• I have updated the tests accordingly. (required for a bug fix or a new feature)
• I have updated the documentation accordingly.
• I have reformatted the code using make format. (required)
• I have checked the code using make lint. (required)
• I have ensured make test pass. (required)