[goal] 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#1886298
- bug#1886296
Bump the lower constraints for required deps which added python3.8
support in their later version.

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: #40193

Closes-Bug: #1886298
Closes-Bug: #1886296

[1] https://governance.openstack.org/tc/goals/selected/victoria/migrate-ci-cd-jobs-to-ubuntu-focal
Change-Id: I3c535fc21dddce6e07817c9af8016fb407d9ed31

lower-constraints.txt

@@ -3,7 +3,7 @@ appdirs==1.4.3
 asn1crypto==0.24.0
 Babel==2.3.4
 certifi==2018.1.18
-cffi==1.11.5
+cffi==1.14
 chardet==3.0.4
 cliff==2.11.0
 cmd2==0.8.1
@@ -66,7 +66,7 @@ pep8==1.5.7
 Pint==0.8.1
 prettytable==0.7.2
 pycparser==2.18
-pyflakes==0.8.1
+pyflakes==2.1.1
 Pygments==2.2.0
 pyinotify==0.9.6
 pymongo==3.6.1

test-requirements.txt

@@ -8,6 +8,11 @@
 #
 # Hacking should appear first in case something else depends on pep8
 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
+
 coverage!=4.4,>=4.0 # Apache-2.0
 nodeenv>=0.9.4 # BSD
 pytest>=5.3.5 # MIT