Use pre-release version of sphinx-a4doc for python 3.13

Author: clonker

docs/requirements.txt

@@ -4,7 +4,9 @@
 sphinx_rtd_theme>=0.5.2
 
 pygments-lexer-solidity>=0.7.0
-sphinx-a4doc>=1.6.0
+sphinx-a4doc>=1.6.0; python_version < '3.13'
+# todo remove this once there is a version > 1.6.0
+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