Updates for py312 compatibility

- Added py312 section to tox.ini [testenv]
- Updated flake8 to 7.1.1 instead of 3.9.2

Change-Id: I097f2434c5adc1126233d86ea80b2bacc7ba4d1e

tox.ini

@@ -56,6 +56,13 @@ deps =
     -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 
+[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}/requirements.txt
+       -r{toxinidir}/test-requirements.txt
+
 [testenv:py3]
 basepython = python3
 deps =
@@ -65,7 +72,7 @@ deps =
 
 [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} hooks unit_tests tests actions lib files
            charm-proof