gh-76785: Print the Traceback from Interpreter.run() (gh-110322)

This is a temporary solution.  The full fix may involve serializing the traceback in some form.

(FYI, I merged this yesterday and the reverted it due to buildbot failures.  See gh-110248.)

Author: ericsnowcurrently

Modules/_xxsubinterpretersmodule.c

@@ -450,7 +450,13 @@ _run_script(PyInterpreterState *interp, const char *codestr,
                 "RunFailedError: script raised an uncaught exception (%s)",
                 failure);
     }
-    Py_XDECREF(excval);
+    if (excval != NULL) {
+        // XXX Instead, store the rendered traceback on sharedexc,
+        // attach it to the exception when applied,
+        // and teach PyErr_Display() to print it.
+        PyErr_Display(NULL, excval, NULL);
+        Py_DECREF(excval);
+    }
     if (errcode != ERR_ALREADY_RUNNING) {
         _PyInterpreterState_SetNotRunningMain(interp);
     }