Remove Sphinx pinning

This was pinned in #434 because of the RTD theme, but #485 switched to
the pydata sphinx theme, so there should not be any issues.

Fixes #435

Author: QuLogic

pyproject.toml

@@ -46,7 +46,7 @@ Homepage = "https://github.com/zarr-developers/numcodecs"
 
 [project.optional-dependencies]
 docs = [
-    "sphinx<7.0.0",
+    "sphinx",
     "sphinx-issues",
     "pydata-sphinx-theme",
     "numpydoc",