Unpin sphinx

[sondreso]

An excpetion occours when using sphinx version 7.2.0.

Extension error (furo):
Handler <function _html_page_context at 0x7f13bf7c25c0> for event 'html-page-context' \
  threw an exception (exception: This documentation is not using `furo.css` as the stylesheet. \
  If you have set `html_style` in your conf.py file, remove it.)

Log example: https://github.com/equinor/ert/actions/runs/5887768050/job/15967783728?pr=5890

Possible incompatability with Furo theme.

[oysteoh]

Could we have a look at this and if it is possible to unpin sphinx at this moment or if there is still an issue with it?