Update sphinx requirement from <8 to <9

Updates the requirements on [sphinx](https://github.com/sphinx-doc/sphinx) to permit the latest version.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v0.1.61611...v8.1.3)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

docs/requirements.txt

@@ -9,7 +9,7 @@ netCDF4
 # actual documentation requirements
 ipykernel
 nbsphinx
-sphinx<8
+sphinx<9
 sphinx-rtd-theme
 sphinx-design
 sphinx-gallery

pyproject.toml

@@ -196,7 +196,7 @@ test = [
 ]
 
 docs = [
-    "sphinx<8",
+    "sphinx<9",
     "nbsphinx",
     "sphinx-rtd-theme",
     "sphinx-design",