Skip pdb test for #13120 if threading is not available.

Lib/test/test_pdb.py

@@ -668,6 +668,9 @@ class PdbTestCase(unittest.TestCase):
         # 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