[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: #40227

Closes-Bug: #1886298

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

Change-Id: I56837a6877a22020048b095b9eb528700c786c66

lower-constraints.txt

@@ -2,12 +2,12 @@ alabaster==0.7.10
 appdirs==1.4.3
 asn1crypto==0.23.0
 certifi==2018.1.18
-cffi==1.7.0
+cffi==1.14.0
 chardet==3.0.4
 cliff==2.11.0
 cmd2==0.8.2
 coverage==4.0
-cryptography==2.1
+cryptography==2.7
 debtcollector==1.19.0
 decorator==4.2.1
 deprecation==2.0
@@ -32,7 +32,6 @@ linecache2==1.0.0
 MarkupSafe==1.0
 mccabe==0.2.1
 monotonic==1.4
-mox3==0.20.0
 msgpack-python==0.4.0
 munch==2.2.0
 netaddr==0.7.19
@@ -65,7 +64,7 @@ python-dateutil==2.5.3
 python-mimeparse==1.6.0
 python-subunit==1.0.0
 pytz==2018.3
-PyYAML==3.12
+PyYAML==3.13
 requests==2.18.4
 requestsexceptions==1.4.0
 rfc3986==0.3.1

requirements.txt

@@ -10,4 +10,4 @@ oslo.utils>=3.36.0 # Apache-2.0
 pbr!=2.1.0,>=3.1.1 # Apache-2.0
 keystoneauth1>=3.4.0 # Apache-2.0
 six>=1.11.0 # MIT
-PyYAML>=3.12 # MIT
+PyYAML>=3.13 # MIT