Update jobs to fix the CI

- remove old / deprecated zuul jobs
- update to the latest rolling python3 jobs
- update tox based on tox4 fixes
- update tox python targets

Change-Id: If77436f4c27cf719907a2888d5b1e9e6c7eb3f7c

.zuul.yaml

@@ -1,5 +1,4 @@
 - project:
     templates:
-      - python35-charm-jobs
-      - openstack-python3-ussuri-jobs
+      - openstack-python3-charm-jobs
       - openstack-cover-jobs

tox.ini

@@ -7,29 +7,26 @@ sitepackages = False
 skip_missing_interpreters = False
 
 [testenv]
+# We use tox mainly for virtual environment management for test requirements
+# and do not install the charm code as a Python package into that environment.
+# Ref: https://tox.wiki/en/latest/config.html#skip_install
+skip_install = True
 setenv = VIRTUAL_ENV={envdir}
          PYTHONHASHSEED=0
-install_command =
-  pip install {opts} {packages}
 commands = stestr run {posargs}
 
-[testenv:py34]
-basepython = python3.4
+[testenv:py38]
+basepython = python3.8
 deps = -r{toxinidir}/test-requirements.txt
 commands = stestr run {posargs}
 
-[testenv:py35]
-basepython = python3.5
+[testenv:py310]
+basepython = python3.10
 deps = -r{toxinidir}/test-requirements.txt
 commands = stestr run {posargs}
 
-[testenv:py36]
-basepython = python3.6
-deps = -r{toxinidir}/test-requirements.txt
-commands = stestr run {posargs}
-
-[testenv:py37]
-basepython = python3.7
+[testenv:py311]
+basepython = python3.11
 deps = -r{toxinidir}/test-requirements.txt
 commands = stestr run {posargs}