CI: Fix up the testing matrix

- Remove py310 from experimental pipeline; it has become the new
  lower-bound for all of OpenStack.
- Add py39 to experimental; we still want to support it, even if
  OpenStack at large has moved on.
- Get rid of py312 overrides; they are no longer necessary.

Change-Id: I64f8dadf661217fde3066f790915c835293c8029

.zuul.yaml

@@ -43,7 +43,8 @@
         - openstack-tox-py36
         - openstack-tox-py37
         - openstack-tox-py38
-        - openstack-tox-py310
+        - openstack-tox-py39
+        - openstack-tox-py311
     check:
       jobs:
         - swiftclient-functional:
@@ -55,18 +56,12 @@
             irrelevant-files: *functest-irrelevant-files
         - tempest-full-py3:
             irrelevant-files: *functest-irrelevant-files
-        - openstack-tox-py311
-        - openstack-tox-py312:
-            voting: true
         - openstack-tox-py313:
             voting: true
     gate:
       jobs:
         - swiftclient-swift-functional
         - swiftclient-functional
-        - openstack-tox-py311
-        - openstack-tox-py312:
-            voting: true
         - openstack-tox-py313:
             voting: true
     post:

tox.ini

@@ -24,7 +24,7 @@ passenv = SWIFT_*
 commands =
     python -m flake8 swiftclient test
 
-[testenv:{py36,py37}]
+[testenv:{py36,py37,py38}]
 # Drop the use of constraints; most dependencies have dropped support for
 # these versions already, and have updated their metadata to reflect that
 deps = -r{toxinidir}/requirements.txt