Use pickle protocol compatible with Python 3.6

Windows builds now use Windows Python 3.9, but
the generated pickles are read by Rocky 8's
Python 3, which is 3.6, so don't exceed the
newest protocol supported by 3.6 (protocol 4).

modules/test/pyext/src/__init__.py

@@ -787,10 +787,13 @@ def __init__(self, stream=None, descriptions=None, verbosity=None):
 
     def stopTestRun(self):
         if 'IMP_TEST_DETAIL_DIR' in os.environ:
+            # Various parts of the IMP build pipeline use Python 3.6,
+            # which predates pickle protocol 5
+            protocol = min(pickle.HIGHEST_PROTOCOL, 4)
             fname = (Path(os.environ['IMP_TEST_DETAIL_DIR'])
                      / Path(sys.argv[0]).name)
             with open(str(fname), 'wb') as fh:
-                pickle.dump(self.all_tests, fh, -1)
+                pickle.dump(self.all_tests, fh, protocol)
         super(_TestResult, self).stopTestRun()
 
     def startTest(self, test):