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.

Story: #2007865
Task: #40183

Closes-Bug: #1886298

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

Change-Id: I5c7bbd9bd43f507a5e45901be10110f2a96f5433

lower-constraints.txt

@@ -39,7 +39,7 @@ linecache2==1.0.0
 logilab-common==1.4.1
 lxml==4.1.1
 Mako==1.0.7
-MarkupSafe==1.0
+MarkupSafe==1.1.1
 mccabe==0.2.1
 monotonic==1.4
 mox3==0.25.0
@@ -86,7 +86,7 @@ python-novaclient==10.1.0
 python-openstackclient==3.14.0
 python-subunit==1.2.0
 pytz==2018.3
-PyYAML==3.12
+PyYAML==3.13
 repoze.lru==0.7
 requests==2.18.4
 requestsexceptions==1.4.0