Reduce strength of error message check.

On some platforms we don't get the whole thing.
https://github.com/python-greenlet/greenlet/actions/runs/6109987277/job/16582093329

src/greenlet/tests/test_cpp.py

@@ -55,7 +55,7 @@ def test_unhandled_std_exception_as_greenlet_function_aborts(self):
         # verify that plain unhandled throw aborts
         output = self._do_test_unhandled_exception('run_as_greenlet_target')
         self.assertIn(
-            'Fatal Python error: g_initialstub: greenlet: Unhandled C++ exception: Thrown from an extension.',
+            'greenlet: Unhandled C++ exception: Thrown from an extension.',
             output
         )