pythongh-93096: Load doctests in test_pickle (pythonGH-131069)

Add doctests to unittest for `pickle`
(cherry picked from commit 3bb20d1)

Co-authored-by: donBarbos <[email protected]>

Author: donbarbos

Lib/test/test_pickle.py

@@ -700,7 +700,7 @@ def test_multiprocessing_exceptions(self):
 
 
 def load_tests(loader, tests, pattern):
-    tests.addTest(doctest.DocTestSuite())
+    tests.addTest(doctest.DocTestSuite(pickle))
     return tests