Update the olddeps CI check to use an old version of markupsafe (#12025)

changelog.d/12025.misc

@@ -0,0 +1 @@
+Update the `olddeps` CI job to use an old version of `markupsafe`.

tox.ini

@@ -124,6 +124,9 @@ usedevelop = false
 deps =
     Automat == 0.8.0
     lxml
+    # markupsafe 2.1 introduced a change that breaks Jinja 2.x. Since we depend on
+    # Jinja >= 2.9, it means this test suite will fail if markupsafe >= 2.1 is installed.
+    markupsafe < 2.1
     {[base]deps}
 
 commands =