handle panic if str is not defined in unraisablehook (RustPython#4864)

Author: itsankitkp

extra_tests/snippets/syntax_del.py

@@ -19,3 +19,13 @@ class MyObject: pass
 
 with assert_raises(NameError):
     del y  # noqa: F821
+
+# see https://github.com/RustPython/RustPython/issues/4863
+
+class MyTest:
+    def __del__(self):
+        type(self)()
+
+def test_del_panic():
+    mytest = MyTest()
+    del mytest

vm/src/stdlib/sys.rs

@@ -590,7 +590,14 @@ mod sys {
     fn unraisablehook(unraisable: UnraisableHookArgs, vm: &VirtualMachine) {
         if let Err(e) = _unraisablehook(unraisable, vm) {
             let stderr = super::PyStderr(vm);
-            writeln!(stderr, "{}", e.as_object().repr(vm).unwrap().as_str());
+            writeln!(
+                stderr,
+                "{}",
+                e.as_object()
+                    .repr(vm)
+                    .unwrap_or_else(|_| vm.ctx.empty_str.to_owned())
+                    .as_str()
+            );
         }
     }