objtype: Set __doc__ as None if not included

This set __doc__ as None if it is not included in attribute
while making new instance of type.
In cpython if there is no docstring in class definition,
__doc__ attribute will be None. So RustPython should follow it.

Partially solves RustPython#1523

Author: seeeturtle

vm/src/obj/objtype.rs

@@ -321,7 +321,12 @@ fn type_new_slot(metatype: PyClassRef, args: PyFuncArgs, vm: &VirtualMachine) ->
         bases
     };
 
-    let attributes = dict.to_attributes();
+    let mut attributes = dict.to_attributes();
+
+    // insert __doc__ as None if it is not included in attributes
+    if !attributes.contains_key("__doc__") {
+        attributes.insert("__doc__".to_string(), vm.ctx.none());
+    }
 
     let mut winner = metatype.clone();
     for base in &bases {