Improve TypedDict conversion logic for shadowed builtins and dunder methods (#5038)

## Summary

This PR (1) avoids flagging `TypedDict` and `NamedTuple` conversions
when attributes are dunder methods, like `__dict__`, and (2) avoids
flagging the `A003` shadowed-attribute rule for `TypedDict` classes at
all, where it doesn't really apply (since those attributes are only
accessed via subscripting anyway).

Closes #5027.

crates/ruff_python_ast/src/helpers.rs

@@ -546,7 +546,7 @@ where
     body.iter().any(|stmt| any_over_stmt(stmt, func))
 }
 
-fn is_dunder(id: &str) -> bool {
+pub fn is_dunder(id: &str) -> bool {
     id.starts_with("__") && id.ends_with("__")
 }