[tox] update pytest dependency, v8.1.0 is yanked

tox.ini

@@ -50,7 +50,7 @@ commands =
 [testenv:examples]
 envdir={toxworkdir}/{envname}
 deps =
-    pytest>=8.1.0,<9.0.0
+    pytest>=7.3.1,<9.0.0
     pytest-xdist>=3.3.1  # multiprocessing
     pytest-forked>=1.6.0  # isolation
     nbmake>=1.5.1