[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.

Depends-On: https://review.opendev.org/#/c/734700/

Story: #2007865
Task: #40207

Closes-Bug: #1886298

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

Change-Id: I14a1d33f1aedfd7bff265b7df2d3aa4340221144

lower-constraints.txt

@@ -11,7 +11,7 @@ eventlet==0.18.2
 extras==1.0.0
 fixtures==3.0.0
 flake8==2.2.4
-greenlet==0.4.10
+greenlet==0.4.15
 hacking==0.10.0
 imagesize==0.7.1
 iso8601==0.1.11
@@ -31,7 +31,7 @@ Pygments==2.2.0
 python-mimeparse==1.6.0
 python-subunit==1.0.0
 pytz==2013.6
-PyYAML==3.12
+PyYAML==3.13
 requests==2.14.2
 requestsexceptions==1.2.0
 restructuredtext-lint==1.1.1