migrate testing to ubuntu focal

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

Fixing:
- bug#1886296
Bump the pyflakes to 2.1.1 as min version to run pep8 jobs
on py3.8 which is default python vesion in ubuntu focal.

Story: #2007865
Task: #40180

Closes-Bug: #1886296
[1] https://governance.openstack.org/tc/goals/selected/victoria/migrate-ci-cd-jobs-to-ubuntu-focal.html

Change-Id: I482ac98bc56f0e3cfb8b767f47649da11ed1afab

Author: Ghanshyam Mann

lower-constraints.txt

@@ -11,6 +11,7 @@ os-client-config==1.29.0 # Apache-2.0
 osc-lib==1.12.1 # Apache-2.0
 
 # test-requirements.txt
+pyflakes==2.1.1
 coverage==4.0 # Apache-2.0
 python-subunit==0.0.18 # Apache-2.0/BSD
 oslotest==1.10.0 # Apache-2.0

test-requirements.txt

@@ -3,6 +3,10 @@
 # process, which may cause wedges in the gate later.
 
 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
 
 coverage>=4.0,!=4.4 # Apache-2.0
 python-subunit>=0.0.18 # Apache-2.0/BSD