[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.

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: Id3df43c47a3bdfce317331e1e3e4df775c4b4f32

lower-constraints.txt

@@ -16,8 +16,8 @@ isort==4.3.4
 Jinja2==2.10
 lazy-object-proxy==1.3.1
 linecache2==1.0.0
-lxml==3.4.1
-MarkupSafe==1.0
+lxml==4.5.0
+MarkupSafe==1.1.1
 mccabe==0.2.1
 mock==2.0.0
 openstackdocstheme==2.2.1
@@ -29,7 +29,7 @@ pylint==1.7.1
 python-mimeparse==1.6.0
 python-subunit==1.0.0
 pytz==2013.6
-PyYAML==3.12
+PyYAML==3.13
 reno==3.1.0
 requests==2.14.2
 restructuredtext-lint==1.1.1

requirements.txt

@@ -4,7 +4,7 @@
 
 pbr!=2.1.0,>=2.0.0 # Apache-2.0
 iso8601>=0.1.11 # MIT
-lxml!=3.7.0,>=3.4.1 # BSD
+lxml>=4.5.0 # BSD
 docutils>=0.11 # OSI-Approved Open Source, Public Domain
 sphinx>=2.0.0,!=2.1.0 # BSD
-PyYAML>=3.12 # MIT
+PyYAML>=3.13 # MIT