[goal] Migrate tox based testing to Focal

As per victoria cycle testing runtime and community goal[1]
we need to migrate upstream CI/CD to Ubuntu Focal(20.04).

Fixing pep8 job.

Stable jobs testing stable branch needs to keep running on their supported
distro version which is bionic from stein till ussuri.

Depends-On: https://review.opendev.org/#/c/738328/

[1] https://governance.openstack.org/tc/goals/selected/victoria/migrate-ci-cd-jobs-to-ubuntu-focal.html

Change-Id: I6cc893ee3cb24afbdc73144213f9d059cec54df3
Story: #2007865
Task: #40186

.zuul.yaml

@@ -1,6 +1,7 @@
 - job:
     name: heat-functional-convg-stein
     parent: heat-functional-convg-mysql-lbaasv2-py3
+    nodeset: openstack-single-node-bionic
     override-checkout: stable/stein
     vars:
       branch_override: stable/stein
@@ -8,6 +9,7 @@
 - job:
     name: heat-functional-orig-stein
     parent: heat-functional-orig-mysql-lbaasv2
+    nodeset: openstack-single-node-bionic
     override-checkout: stable/stein
     vars:
       branch_override: stable/stein
@@ -16,6 +18,7 @@
 - job:
     name: heat-functional-convg-train
     parent: heat-functional-convg-mysql-lbaasv2-py3
+    nodeset: openstack-single-node-bionic
     override-checkout: stable/train
     vars:
       branch_override: stable/train
@@ -23,6 +26,7 @@
 - job:
     name: heat-functional-orig-train
     parent: heat-functional-orig-mysql-lbaasv2
+    nodeset: openstack-single-node-bionic
     override-checkout: stable/train
     vars:
       branch_override: stable/train
@@ -31,6 +35,7 @@
 - job:
     name: heat-functional-ussuri
     parent: heat-functional
+    nodeset: openstack-single-node-bionic
     override-checkout: stable/ussuri
 
 - project:

test-requirements.txt

@@ -4,4 +4,7 @@
 
 # Hacking already pins down pep8, pyflakes and flake8
 hacking!=0.13.0,<0.14,>=0.12.0 # Apache-2.0
-
+# remove this pyflakes from here once you bump the
+# hacking to 3.2.0 or above. hacking 3.2.0 takes
+# care of pyflakes version compatibilty.
+pyflakes>=2.1.1