Support Python 3.10

Add a CI job to run tests on Python 3.10, which is supported
in 2023.2 [0].

As rbd-iscsi-client is on an independent release model, it
still supports Python 3.8, so adjust .zuul.yaml and tox.ini
to use the 2023.1 upper constraints when running py38 tests.

[0] https://governance.openstack.org/tc/reference/runtimes/2023.2.html

Change-Id: I7fa0189dac8d980aa4fd5a7c8bac2f6f84ceb6d5

.zuul.yaml

@@ -12,8 +12,9 @@
       jobs:
         - openstack-tox-pep8
         - rbd-iscsi-client-py36
-        - openstack-tox-py38
+        - rbd-iscsi-client-py38
         - openstack-tox-py39
+        - openstack-tox-py310
         - rbd-iscsi-client-code-coverage:
             voting: false
 
@@ -21,8 +22,9 @@
       jobs:
         - openstack-tox-pep8
         - rbd-iscsi-client-py36
-        - openstack-tox-py38
+        - rbd-iscsi-client-py38
         - openstack-tox-py39
+        - openstack-tox-py310
 
 - job:
     name: rbd-iscsi-client-code-coverage
@@ -47,3 +49,13 @@
     required-projects:
       - name: openstack/requirements
         override-checkout: stable/yoga
+
+- job:
+    name: rbd-iscsi-client-py38
+    parent: openstack-tox-py38
+    # NOTE: zuul ignores TOX_CONSTRAINTS_FILE and uses upper constraints
+    # directly from the requirements repo, so we need to tell it to use
+    # the u-c from a branch that still supports py38
+    required-projects:
+      - name: openstack/requirements
+        override-checkout: stable/2023.1

setup.cfg

@@ -23,6 +23,7 @@ classifiers =
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
+    Programming Language :: Python :: 3.10
 
 [global]
 setup-hooks =

tox.ini

@@ -38,14 +38,21 @@ passenv =
     no_proxy
     NO_PROXY
 
-[testenv:py36]
-# We have to override the tox default py36 testenv because we need to use
-# upper constraints from the last openstack release that supports python 3.6
+[testenv:py{36,37}]
+# The last openstack release to support python 3.6 and 3.7 is yoga, so we
+# need to use the yoga upper constraints
 deps =
     -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/yoga}
     -r{toxinidir}/test-requirements.txt
     -r{toxinidir}/requirements.txt
 
+[testenv:py38]
+# The last openstack release to support python 3.8 is 2023.1 (Antelope)
+deps =
+    -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/2023.1}
+    -r{toxinidir}/test-requirements.txt
+    -r{toxinidir}/requirements.txt
+
 [testenv:pep8]
 commands = flake8 {posargs}