Update sageinspect.getargspec to handle keyword only arguments

[Biffo89]

Fixes #39627

Keyword-only arguments were added in Python 3 and are not being handled properly by sageinspect, causing documentation to have incorrect signatures, including incorrect default arguments.

This PR replaces a usage of the deprecated inspect.getargs and inspect.FullArgSpec with inspect.getfullargspec.

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies

[tobiasdiez]

Thanks a lot for your PR.

There are a few more calls to inspect.getargs further down in the same method. Could you please replace them as well? (Looking at the code, it appears to me that func_obj is always obj, so the first call with inspect.getargs(func_obj.__code__) is actually a duplicate and can be removed.)

Finally, if you have time, could you please try if inspect.signature works better/the same than getfullargspec. The latter is scheduled for deprecation, see python/cpython#108901 (although this seems to lie dormant for some time now).