Fix issues related to py312 compatibility

Summary of changes:
- Bump flake8 from 3.9.2 to 7.1.1
- Add py312 section to tox.ini [testenv]
- Updated zuul.yaml to use openstack-python3-charm-jobs

Change-Id: Icf8578cb9717615afee119640e1f0437770c396f

.zuul.yaml

@@ -1,4 +1,4 @@
 - project:
     templates:
-      - openstack-python3-charm-yoga-jobs
+      - openstack-python3-charm-jobs
       - openstack-cover-jobs

tox.ini

@@ -72,9 +72,14 @@ basepython = python3.10
 deps = -r{toxinidir}/test-requirements.txt
 commands = stestr run --slowest {posargs}
 
+[testenv:py312]
+basepython = python3.12
+deps = -r{toxinidir}/test-requirements.txt
+commands = stestr run --slowest {posargs}
+
 [testenv:pep8]
 basepython = python3
-deps = flake8==3.9.2
+deps = flake8==7.1.1
        git+https://github.com/juju/charm-tools.git
 commands = flake8 {posargs} src unit_tests