Add support for sphinx 9

[cooperlees]

Hello,

Can we get furo happy with sphinx 9 plz. Was not sure to come here or an issue.

The conflict is caused by:
    The user requested sphinx==9.0.0
    furo 2025.9.25 depends on sphinx<9.0 and >=6.0

Thanks,
Cooper

[JohnVillalovos]

FYI already discussed in #912

[cooperlees]

Don't know how I missed this - I did look :| - Closing for #912