bump docs building to python3.13 to meet Sphinx minimum dependency

Author: Kriechi

pyproject.toml

@@ -150,10 +150,10 @@ env_list = [ "py310", "py311", "py312", "py313", "py314", "lint", "docs", "packa
 
 [tool.tox.gh-actions]
 python = """
-  3.10: py310, h2spec, lint, docs, packaging
+  3.10: py310, h2spec, lint, packaging
   3.11: py311
   3.12: py312
-  3.13: py313
+  3.13: py313, docs
   3.14: py314
 """