python3 might not be installed in a non-primary python environment
(15.4_py39 backports)

Index: pegasus-wms.common-gh-5.0.3/test/client/test_client.py
===================================================================
--- pegasus-wms.common-gh-5.0.3.orig/test/client/test_client.py
+++ pegasus-wms.common-gh-5.0.3/test/client/test_client.py
@@ -5,6 +5,7 @@ import subprocess
 from collections import namedtuple
 from pathlib import Path
 from subprocess import Popen
+import sys
 from tempfile import TemporaryDirectory
 from textwrap import dedent
 
@@ -81,7 +82,7 @@ class TestClient:
 
         # fork process to print 0\n1\n..4\n"
         proc = Popen(
-            ["python3", "-c", 'exec("for i in range(5):\\n\\tprint(i)\\n")'],
+            [sys.executable, "-c", 'exec("for i in range(5):\\n\\tprint(i)\\n")'],
             stdout=subprocess.PIPE,
             stderr=subprocess.PIPE,
         )
@@ -109,7 +110,7 @@ class TestClient:
 
         # fork process to print 0\n1\n..4\n"
         proc = Popen(
-            ["python3", "-c", 'exec("for i in range(5):\\n\\tprint(i)\\n")'],
+            [sys.executable, "-c", 'exec("for i in range(5):\\n\\tprint(i)\\n")'],
             stdout=subprocess.PIPE,
             stderr=subprocess.PIPE,
         )
@@ -132,7 +133,7 @@ class TestClient:
 
         # fork process to print 0\n1\n..4\n"
         proc = Popen(
-            ["python3", "-c", 'exec("for i in range(5):\\n\\tprint(i)\\n")'],
+            [sys.executable, "-c", 'exec("for i in range(5):\\n\\tprint(i)\\n")'],
             stdout=subprocess.PIPE,
             stderr=subprocess.PIPE,
         )