With AIX I have seen the following messages repeatedly:

root@x066:[/data/prj/python/python3-3.8]./python ../git/*Lib/ctypes/test/test_loading.py
/data/prj/python/git/python3-3.8/Lib/subprocess.py:852: ResourceWarning: subprocess 11796734 is still running
  ResourceWarning, source=self)
/data/prj/python/git/python3-3.8/Lib/subprocess.py:852: ResourceWarning: subprocess 11796480 is still running
  ResourceWarning, source=self)

There is python2 to python3 missed change typo that is preventing the returncode from ever being determined

diff --git a/Lib/ctypes/_aix.py b/Lib/ctypes/_aix.py
index 463f60a284..190cac6507 100644
--- a/Lib/ctypes/_aix.py
+++ b/Lib/ctypes/_aix.py
@@ -115,7 +115,7 @@ def get_ld_headers(file):
         else:
             break
     p.stdout.close()
-    p.wait
+    p.wait()
     return ldr_headers

 def get_shared(ld_headers):

PR will be posted.