build: test light-threads on all versions of Python

And also use more compact tox environment syntax throughout tox.init

Author: nedbat

tox.ini

@@ -20,7 +20,7 @@ extras =
 deps =
     -r requirements/pip.pip
     -r requirements/pytest.pip
-    py3{10,11}: -r requirements/light-threads.pip
+    py3{10,11,12,13,14}: -r requirements/light-threads.pip
 
 # Windows can't update the pip version with pip running, so use Python
 # to install things.