Allow None as with_tracebak arg

tests/snippets/stdlib_traceback.py

@@ -15,3 +15,13 @@ try:
 except KeyError as ex2:
 	tb = traceback.extract_tb(ex2.__traceback__)
 	assert tb[1].line == "1/0"
+
+
+try:
+	try:
+		1/0
+	except ZeroDivisionError as ex:
+		 raise ex.with_traceback(None)
+except ZeroDivisionError as ex2:
+	tb = traceback.extract_tb(ex2.__traceback__)
+	assert len(tb) == 1

vm/src/exceptions.rs

@@ -210,10 +210,14 @@ fn exception_repr(vm: &VirtualMachine, args: PyFuncArgs) -> PyResult {
 
 fn exception_with_traceback(
     zelf: PyObjectRef,
-    tb: PyTracebackRef,
+    tb: Option<PyTracebackRef>,
     vm: &VirtualMachine,
 ) -> PyResult {
-    vm.set_attr(&zelf, "__traceback__", tb)?;
+    vm.set_attr(
+        &zelf,
+        "__traceback__",
+        tb.map_or(vm.get_none(), |tb| tb.into_object()),
+    )?;
     Ok(zelf)
 }