build: switch to python3

Refs: #5095
Change-Id: Iedc107d41d4832682071986678b015980354786d
diff --git a/.jenkins.d/20-tests.sh b/.jenkins.d/20-tests.sh
index ec4461b..9c30a19 100755
--- a/.jenkins.d/20-tests.sh
+++ b/.jenkins.d/20-tests.sh
@@ -15,7 +15,7 @@
 
 ndnsec-keygen "/tmp/jenkins/$NODE_NAME" | ndnsec-install-cert -
 
-BOOST_VERSION=$(python -c "import sys; sys.path.append('build/c4che'); import _cache; print(_cache.BOOST_VERSION_NUMBER);")
+BOOST_VERSION=$(python3 -c "import sys; sys.path.append('build/c4che'); import _cache; print(_cache.BOOST_VERSION_NUMBER);")
 
 ut_log_args() {
     if (( BOOST_VERSION >= 106200 )); then