Fix tox pep8 job for k8sapp_openstack

The Tox job for pep8 is failing for k8sapp_openstack due to a missing
file in the openstack/requirements repository [1]. This job fetches the
upper constraints from the Stein branch, but it no longer exists.

This commit simply updates the upper constraints [1] branch from
"branch/stable/stein" to "tag/stein-eol". Currently, there's no way to
set this value to a more recent branch (stable/2023.1 for example),
because the packages for these newer versions do not support Python 3.6,
and Tox is failing to use Python 3.9 version for the Pep8 task.

[1] https://opendev.org/openstack/requirements

Test Plan:
  - PASS: Run tox for the python3-k8sapp-openstack folder

Closes-Bug: 2077730

Change-Id: Ic65f1b840098fb048fe9f2ce00d1e33a4a58c557
Signed-off-by: Lucas de Ataides <lucas.deataidesbarreto@windriver.com>

python3-k8sapp-openstack/k8sapp_openstack/tox.ini

@@ -102,7 +102,7 @@ deps = -r{toxinidir}/test-requirements.txt
        flake8-import-order
 install_command = pip install -v -v -v \
     -c{toxinidir}/upper-constraints.txt \
-    -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/stable/stein/upper-constraints.txt} \
+    -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/tag/stein-eol/upper-constraints.txt} \
     {opts} {packages}
 commands =
   flake8 {posargs} .
@@ -191,4 +191,4 @@ install_command = pip install -v -v -v \
     {opts} {packages}
 # Pass top level app folder to 'sysinv-app tox' command.
 commands =
-  bash -c "echo $(dirname $(dirname $(pwd))) | xargs -n 1 sysinv-app tox"
+  bash -c "echo $(dirname $(dirname $(pwd))) | xargs -n 1 sysinv-app tox"