CI: fix lower-constraints job

This is a combination of 2 commits.

----------------------------------------

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.

----------------------------------------

CI: Use [tox]requires to pin virtualenv

Using an antiquated distro to keep us on an EOL'ed version of python
that *just so happens* to have had support dropped for it around the
same time virtualenv removed the ability to create py27 environments
doesn't seem like the right way to address the problem.

Make sure we also include the tox<4 pin that we're using project-wide
so we don't get some nonsense about "Multiple top-level packages
discovered in a flat-layout".

Related-Change: https://review.opendev.org/c/openstack/swift/+/881035
Related-Change: I32a161e555179ca34d306ac37e4097611853e36b

----------------------------------------

Change-Id: Ia2b3cc4f719347705dec15f1fc97d982e72bfb34

.zuul.yaml

@@ -482,6 +482,8 @@
 - job:
     name: swift-tox-lower-constraints
     parent: openstack-tox-lower-constraints
+    # This seems defensible for a l-c job
+    nodeset: ubuntu-jammy
     vars:
       bindep_profile: test py27
       python_version: 2.7

bindep.txt

@@ -17,7 +17,7 @@ libxslt-devel [platform:rpm]
 libxslt1-dev [platform:dpkg]
 libxslt-dev [platform:apk]
 memcached
-python-dev [platform:dpkg platform:apk]
+python-dev [platform:dpkg platform:apk !platform:ubuntu-jammy]
 python-devel [platform:rpm !py36]
 python3-dev [platform:dpkg platform:apk test]
 python3-devel [platform:rpm !py27 test]

tox.ini

@@ -2,6 +2,11 @@
 envlist = py37,py27,pep8
 minversion = 2.3.2
 skipsdist = True
+requires =
+  # required to support py27/py36 envs
+  virtualenv<20.22
+  # project-wide requirement; see .zuul.yaml
+  tox<4
 
 [testenv]
 usedevelop = True