[goal] Migrate tox based 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: #40177
Depends-On: https://review.opendev.org/#/c/738328/

Change-Id: I046e2bf0e730849ef6fd8b8d1d355916632a467e

lower-constraints.txt

@@ -1,11 +1,11 @@
 appdirs==1.3.0
 asn1crypto==0.23.0
 Babel==2.3.4
-cffi==1.7.0
+cffi==1.14.0
 cliff==2.8.0
 cmd2==0.8.0
 coverage==4.1
-cryptography==2.1
+cryptography==2.7
 debtcollector==1.2.0
 decorator==3.4.0
 deprecation==1.0
@@ -57,7 +57,7 @@ python-novaclient==9.1.0
 python-openstackclient==3.12.0
 python-subunit==1.0.0
 pytz==2013.6
-PyYAML==3.12
+PyYAML==3.13
 requests==2.14.2
 requests-mock==1.2.0
 requestsexceptions==1.2.0