Update TOX_CONSTRAINTS_FILE for stable/xena

Update the URL to the upper-constraints file to point to the redirect
rule on releases.openstack.org so that anyone working on this branch
will switch to the correct upper-constraints list automatically when
the requirements repository branches.

Until the requirements repository has as stable/xena branch, tests will
continue to use the upper-constraints list on master.

Change-Id: I76573020a2a9ed503b1a35faff3e37a5b0561cd0

.gitreview

@@ -2,4 +2,4 @@
 host=review.opendev.org
 port=29418
 project=openstack/python-watcherclient.git
-defaultbranch=stable/zed
+defaultbranch=stable/xena

.zuul.yaml

@@ -1,7 +1,7 @@
 - project:
     templates:
       - openstack-cover-jobs
-      - openstack-python3-zed-jobs
+      - openstack-python3-xena-jobs
       - publish-openstack-docs-pti
       - check-requirements
       - openstackclient-plugin-jobs

setup.cfg

@@ -6,7 +6,7 @@ description_file =
 author = OpenStack
 author_email = openstack-discuss@lists.openstack.org
 home_page =  https://docs.openstack.org/python-watcherclient/latest/
-python_requires = >=3.8
+python_requires = >=3.6
 classifier =
     Environment :: OpenStack
     Intended Audience :: Information Technology
@@ -15,8 +15,9 @@ classifier =
     Operating System :: POSIX :: Linux
     Programming Language :: Python
     Programming Language :: Python :: 3
+    Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
-    Programming Language :: Python :: 3.9
 
 [files]
 packages =

tox.ini

@@ -11,7 +11,7 @@ install_command = pip install {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
 deps =
-  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/zed}
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/xena}
   -r{toxinidir}/test-requirements.txt
   -r{toxinidir}/requirements.txt
 allowlist_externals =
@@ -43,7 +43,7 @@ commands =
 [testenv:docs]
 basepython = python3
 deps =
-  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/zed}
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/xena}
   -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -W -b html doc/source doc/build/html