migrate tox based 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.

Integratrion testing migration will be done later once
https://storyboard.openstack.org/#!/story/2007732 is fixed

Story: #2007865
Task: #40177

[1] https://governance.openstack.org/tc/goals/selected/victoria/migrate-ci-cd-jobs-to-ubuntu-focal.html
Change-Id: Ic4fd981240f59da4106088c61baec801e031f2c3

barbican/plugin/dogtag.py

@@ -666,7 +666,7 @@ def _catch_subca_deletion_exceptions(ca_related_function):
     def _catch_subca_exception(self, *args, **kwargs):
         try:
             return ca_related_function(self, *args, **kwargs)
-        except pki.ResourceNotFoundException as e:
+        except pki.ResourceNotFoundException:
             LOG.warning("Sub-CA already deleted")
             pass
         except pki.PKIException as e:

test-requirements.txt

@@ -4,6 +4,7 @@
 
 # hacking should appear first in case something else depends on pep8
 hacking!=0.13.0,<0.14,>=0.12.0 # Apache-2.0
+pyflakes>=2.1.1
 
 coverage!=4.4,>=4.0 # Apache-2.0
 ddt>=1.0.1 # MIT