[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
Bump the lower constraints for required deps which added python3.8 support
in their later version.
- 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: #40229

Change-Id: Ia8f69313be9b5b329773ce4a77168cbd44dc300b

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

test-requirements.txt

@@ -8,6 +8,10 @@
 #
 # 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
 testtools>=2.2.0 # MIT