[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: #40190

Closes-Bug: #1886298

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

Change-Id: Ieada47b1455e635208bafe12168fadcb65bc72b9

lower-constraints.txt

@@ -2,7 +2,7 @@ appdirs==1.3.0
 asn1crypto==0.23.0
 Babel==2.3.4
 bandit==1.1.0
-cffi==1.7.0
+cffi==1.14.0
 cliff==2.8.0
 cmd2==0.8.0
 coverage==4.0
@@ -21,7 +21,7 @@ jsonschema==2.6.0
 keyring==5.5.1
 keystoneauth1==3.4.0
 linecache2==1.0.0
-lxml==3.4.1
+lxml==4.5.0
 mccabe==0.2.1
 mock==2.0.0
 monotonic==0.6
@@ -53,7 +53,7 @@ python-dateutil==2.5.3
 python-mimeparse==1.6.0
 python-subunit==1.0.0
 pytz==2013.6
-PyYAML==3.12
+PyYAML==3.13
 requests-mock==1.2.0
 requests==2.14.2
 requestsexceptions==1.2.0

test-requirements.txt

@@ -8,7 +8,7 @@ flake8-docstrings==0.2.1.post1 # MIT
 coverage!=4.4,>=4.0 # Apache-2.0
 fixtures>=3.0.0 # Apache-2.0/BSD
 keyring>=5.5.1 # MIT/PSF
-lxml!=3.7.0,>=3.4.1 # BSD
+lxml>=4.5.0 # BSD
 oauthlib>=0.6.2 # BSD
 os-client-config>=1.28.0 # Apache-2.0
 oslotest>=3.2.0 # Apache-2.0