Add support for Python 3.12

{{cookiecutter.project_slug}}/tox.ini

 [tox]
 # Generative environment list to test all supported Python versions
-envlist = py3{8,9,10,11},black,pep8,pylint
+envlist = py3{8,9,10,11,12},black,pep8,pylint
 minversion = 3.18.0
 
 [testenv]