tox: remove unsupported interpreters for sphinx-develop environments

Sphinx's development tree only supports back to Python v3.10. Dropping
the old environments from the tox configuration.

Signed-off-by: James Knight <james.d.knight@live.com>

tox.ini

@@ -31,7 +31,7 @@ commands =
     {envpython} -m coverage html
     {envpython} -m coverage report
 
-[testenv:{,py38-,py39-,py310-,py311-,py312-,py313-}develop]
+[testenv:{,py310-,py311-,py312-,py313-}develop]
 deps =
     {[testenv]deps}
     git+https://github.com/sphinx-doc/sphinx.git@{env:SPHINX_VER:master}