[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.

Also adding bindep file for libffi packages.

Story: #2007865
Task: #40200

Change-Id: I8d9b8d9e19ca7b94f1ce1f2b95aeafc9f7127ea3

bindep.txt

@@ -0,0 +1,5 @@
+# This is a cross-platform list tracking distribution packages needed for install and tests;
+# see https://docs.openstack.org/infra/bindep/ for additional information.
+
+libffi-dev [platform:dpkg test]
+libffi-devel [platform:rpm test]

lower-constraints.txt

@@ -3,7 +3,7 @@ amqp==2.2.2
 appdirs==1.3.0
 Babel==2.5.3
 cachetools==2.0.1
-cffi==1.11.5
+cffi==1.14.0
 contextlib2==0.5.5
 coverage==4.0
 debtcollector==1.19.0
@@ -25,7 +25,7 @@ Jinja2==2.10
 keystoneauth1==3.4.0
 kombu==4.1.0
 linecache2==1.0.0
-MarkupSafe==1.0
+MarkupSafe==1.1.1
 mccabe==0.2.1
 monotonic==1.4
 msgpack-python==0.4.0

tox.ini

@@ -72,3 +72,11 @@ max-complexity = 30
 
 [hacking]
 import_exceptions = os_vif.i18n
+
+[testenv:bindep]
+# Do not install any requirements. We want this to be fast and work even if
+# system dependencies are missing, since it's used to tell you what system
+# dependencies are missing! This also means that bindep must be installed
+# separately, outside of the requirements files.
+deps = bindep
+commands = bindep test