[5.1.x] Made RemoteTestResultTest.test_pickle_errors_detection() compatible with tblib 3.2+.

tblib 3.2+ makes exception subclasses with __init__() and the default
__reduce__() picklable. This broke the test for
RemoteTestResult._confirm_picklable(), which expects a specific
exception to fail unpickling.

https://github.com/ionelmc/python-tblib/blob/master/CHANGELOG.rst#320-2025-10-21

This fix defines ExceptionThatFailsUnpickling.__reduce__() in a way
that pickle.dumps(obj) succeeds, but pickle.loads(pickle.dumps(obj))
raises TypeError.

Refs #27301. This preserves the intent of the regression test from
52188a5ca6 without skipping it.

Backport of 548209e620 from main.

tests/test_runner/test_parallel.py

@@ -21,6 +21,12 @@ class ExceptionThatFailsUnpickling(Exception):
     def __init__(self, arg):
         super().__init__()
 
+    def __reduce__(self):
+        # tblib 3.2+ makes exception subclasses picklable by default.
+        # Return (cls, ()) so the constructor fails on unpickle, preserving
+        # the needed behavior for test_pickle_errors_detection.
+        return (self.__class__, ())
+
 
 class ParallelTestRunnerTest(SimpleTestCase):
     """
@@ -109,6 +115,8 @@ class RemoteTestResultTest(SimpleTestCase):
         result = RemoteTestResult()
         result._confirm_picklable(picklable_error)
 
+        # The exception can be pickled but not unpickled.
+        pickle.dumps(not_unpicklable_error)
         msg = "__init__() missing 1 required positional argument"
         with self.assertRaisesMessage(TypeError, msg):
             result._confirm_picklable(not_unpicklable_error)