Pin pip<21.0 for running tests with Python 2

"pip 21.0, in January 2021, will remove Python 2 support, per pip’s
Python 2 Support policy. Please migrate to Python 3." from
https://pip.pypa.io/en/stable/

21.0 has been released since Jan 23, 2021.

Signed-off-by: Chenxiong Qi <cqi@redhat.com>

test.sh

@@ -64,7 +64,7 @@ function setup_dfp() {
 
   # CentOS needs to have setuptools updates to make pytest-cov work
   if [[ $OS = "centos" && $OS_VERSION == 7  ]]; then
-    $RUN "${PIP_INST[@]}" -U pip
+    $RUN "${PIP_INST[@]}" -U 'pip<21.0'
     $RUN "${PIP_INST[@]}" -U setuptools
   fi