Tidy tox configurations

Add py312 target, drop use of requirements.txt as this
is not provided in the repository.

Change-Id: I27dff8a210433557e1be3da6145f73fac634f86f

tox.ini

@@ -18,8 +18,6 @@ setenv = VIRTUAL_ENV={envdir}
          TERM=linux
 install_command =
   pip install {opts} {packages}
-deps =
-    -r{toxinidir}/requirements.txt
 
 [testenv:py3]
 basepython = python3
@@ -61,6 +59,11 @@ basepython = python3.11
 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 = -r{toxinidir}/test-requirements.txt