bpo-32294: Fix multiprocessing test_semaphore_tracker() (#4885)

Run the child process with -E option to ignore the PYTHONWARNINGS
environment variable.

Author: vstinner

Lib/test/_test_multiprocessing.py

@@ -4365,7 +4365,7 @@ def test_semaphore_tracker(self):
         '''
         r, w = os.pipe()
         p = subprocess.Popen([sys.executable,
-                             '-c', cmd % (w, w)],
+                             '-E', '-c', cmd % (w, w)],
                              pass_fds=[w],
                              stderr=subprocess.PIPE)
         os.close(w)