relax sphinx version range to [2.1, 9)

Author: clonker

docs/requirements.txt

@@ -9,4 +9,4 @@ sphinx-a4doc>=1.6.0; python_version < '3.13'
 sphinx-a4doc @ git+https://github.com/taminomara/sphinx-a4doc@f63d3b2; python_version >= '3.13'
 
 # Sphinx 2.1.0 is the oldest version that accepts a lexer class in add_lexer()
-sphinx>=2.1.0, <6.0
+sphinx>=2.1.0, <9.0