Update docker image

Switch to ubuntu:20.04 as a base image
to use python3.8 natively and to support the
latest CVE fixes for python packages

Related-Prod: PROD-36143
Change-Id: I5e1e8fb6d3b4987522a96b621898afd08958fe20
diff --git a/test_set/cvp-sanity/requirements.txt b/test_set/cvp-sanity/requirements.txt
index d360de3..71160cc 100644
--- a/test_set/cvp-sanity/requirements.txt
+++ b/test_set/cvp-sanity/requirements.txt
@@ -1,10 +1,10 @@
-pytest==3.10
-pytest-rerunfailures==7.0
-requests==2.24.0
-flake8==3.8.3
-PyYAML==5.3.1
-python-jenkins==1.7.0
-pygerrit2==2.0.9
-GitPython==3.1.7
-ldap3==2.7
+pytest>=3.10
+pytest-rerunfailures>=7.0
+requests>=2.24.0
+flake8>=3.8.3
+PyYAML>=5.4
+python-jenkins>=1.7.0
+pygerrit2>=2.0.9
+GitPython>=3.1.7
+ldap3>=2.7