Skip pdb test for #13120 if threading is not available.
diff --git a/Lib/test/test_pdb.py b/Lib/test/test_pdb.py
index dc2609e..8355e7d 100644
--- a/Lib/test/test_pdb.py
+++ b/Lib/test/test_pdb.py
@@ -668,6 +668,9 @@
         # invoking "continue" on a non-main thread triggered an exception
         # inside signal.signal
 
+        # raises SkipTest if python was built without threads
+        support.import_module('threading')
+
         with open(support.TESTFN, 'wb') as f:
             f.write(textwrap.dedent("""
                 import threading