Change pep8 to run on debian bullseye with python 3.9

All other jobs are already on bullseye,
But the pep8 still runs on ubuntu bionic with python 3.6

The older Ubuntu Bionic boxes are frequently failing to be allocated

Closes-bug: 2121541

Change-Id: Idca3ce6574f45263f3f19dba3df608bd5b3f1f1a
Signed-off-by: caio-volpato <caio.volpato@windriver.com>

.zuul.yaml

@@ -102,7 +102,7 @@
     parent: tox
     description: |
       Run pep8 tests for k8sapp_openstack
-    nodeset: ubuntu-bionic
+    nodeset: debian-bullseye
     required-projects:
       - starlingx/config
       - starlingx/fault

python3-k8sapp-openstack/k8sapp_openstack/tox.ini

@@ -96,13 +96,12 @@ import-order-style = google
 application-import-names = k8sapp_openstack
 
 [testenv:pep8]
-basepython = python3.6
 deps = -r{toxinidir}/test-requirements.txt
        flake8-bugbear
-       flake8-import-order~=0.18.2 # 0.19.* doesn't work with python 3.7 and 3.8 -> https://github.com/PyCQA/flake8-import-order/issues/210
+       flake8-import-order~=0.19.2
 install_command = pip install -v -v -v \
     -c{toxinidir}/upper-constraints.txt \
-    -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/tag/stein-eol/upper-constraints.txt} \
+    -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/starlingx/root/raw/branch/master/build-tools/requirements/debian/upper-constraints.txt} \
     {opts} {packages}
 commands =
   flake8 {posargs} .