bpo-31904: add shell requirement for test_pipes (GH-23489)



VxWorks has no user space shell provided so it can't support pipes module. Also add shell requirement for running test_pipes.
diff --git a/Lib/test/test_pipes.py b/Lib/test/test_pipes.py
index 7d8cd54..6a13b36 100644
--- a/Lib/test/test_pipes.py
+++ b/Lib/test/test_pipes.py
@@ -3,13 +3,16 @@
 import string
 import unittest
 import shutil
-from test.support import run_unittest, reap_children
+from test.support import run_unittest, reap_children, unix_shell
 from test.support.os_helper import TESTFN, unlink
 
 
 if os.name != 'posix':
     raise unittest.SkipTest('pipes module only works on posix')
 
+if not (unix_shell and os.path.exists(unix_shell)):
+    raise unittest.SkipTest('pipes module requires a shell')
+
 TESTFN2 = TESTFN + "2"
 
 # tr a-z A-Z is not portable, so make the ranges explicit