CI: fix lower-constraints job

Switch from focal back to bionic so tox gets installed with py36. This
ensures that we get virtualenv<20.18.0 so we can still create py27
environments (support for which was dropped in virtualenv==20.22.0).

Note that this also means we can drop our tox<4 pin, as newer tox does
not support py36, either.

Change-Id: Ia2b3cc4f719347705dec15f1fc97d982e72bfb34

.zuul.yaml

@@ -526,13 +526,13 @@
 - job:
     name: swift-tox-lower-constraints
     parent: openstack-tox-lower-constraints
+    # This seems defensible for a l-c job
+    nodeset: ubuntu-bionic
     vars:
       bindep_profile: test py27
       python_version: 2.7
       tox_environment:
         TMPDIR: '{{ ansible_env.HOME }}/xfstmp'
-      # This seems defensible for a l-c job
-      ensure_tox_version: '<4'
 
 # Image building jobs
 - secret: