Avoid using typing.Self in stub files pre-Python 3.11 (#14230)

## Summary

See:
#14217 (comment).

This means we're recommending `typing_extensions` in non-stubs pre-3.11,
which may not be a valid project dependency, but that's a separate issue
(#9761).

crates/ruff_linter/src/rules/flake8_pyi/rules/non_self_return_type.rs

@@ -191,7 +191,7 @@ fn add_diagnostic(
     /// Return an [`Edit`] that imports `typing.Self` from `typing` or `typing_extensions`.
     fn import_self(checker: &Checker, range: TextRange) -> Option<Edit> {
         let target_version = checker.settings.target_version.as_tuple();
-        let source_module = if checker.source_type.is_stub() || target_version >= (3, 11) {
+        let source_module = if target_version >= (3, 11) {
             "typing"
         } else {
             "typing_extensions"