Fix pre-installation of universal2 environment for OSX tests

We need to pre-install the same versions that will be installed by
buildout later. For most distributions there is a constraint in
constaints.txt that we keep up to date with buildout.cfg, but not for
MarkupSafe

tox.ini

@@ -34,7 +34,8 @@ deps =
     universal2: AccessControl
     universal2: zodbpickle
     universal2: charset_normalizer
-    universal2: MarkupSafe
+    universal2: MarkupSafe==2.1.5; python_version == '3.8'
+    universal2: MarkupSafe==3.0.1; python_version > '3.8'
     universal2: zope.testrunner
 setenv =
 commands_pre =