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: #40178

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

Change-Id: If5262d4b4c67fddc55125ee3ca9c7f6c34ebade9

lower-constraints.txt

@@ -29,7 +29,7 @@ oslotest==3.2.0
 pbr==2.0.0
 pep8==1.5.7
 prettytable==0.7.1
-pyflakes==0.8.1
+pyflakes==2.1.1
 pyinotify==0.9.6
 pyparsing==2.1.0
 pyperclip==1.5.27

test-requirements.txt

@@ -2,6 +2,11 @@
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
 hacking>=1.1.0,<1.2.0 # 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
+
 mock>=3.0.0 # BSD
 oslotest>=3.2.0 # Apache-2.0
 fixtures>=3.0.0 # Apache-2.0/BSD