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]

Change-Id: Ie91caddf89851bfc5d0de985a5da2d5db93954b9

tox.ini

@@ -66,6 +66,13 @@ deps =
     -r{toxinidir}/test-requirements.txt
 commands = stestr run --slowest {posargs}
 
+[testenv:py312]
+basepython = python3.12
+deps =
+    -c {env:TEST_CONSTRAINTS_FILE:https://raw.githubusercontent.com/openstack-charmers/zaza-openstack-tests/master/constraints/constraints-2024.1.txt}
+    -r{toxinidir}/test-requirements.txt
+commands = stestr run --slowest {posargs}
+
 [testenv:py310]
 basepython = python3.10
 deps =
@@ -75,7 +82,7 @@ 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