Preserve str subclass type returned by __str__ / __repr__ by changjoon-park · Pull Request #7701 · RustPython/RustPython

Closes RustPython#7450.

CPython's unicode_new_impl returns the PyObject_Str result as-is when
type == &PyUnicode_Type, only invoking unicode_subtype_new for actual
str subclasses. RustPython's PyStr::Constructor stripped the result via
Self::from(s.as_wtf8().to_owned()) and re-materialized through
into_ref_with_type, dropping the subclass type even when cls is exactly
str.

Add a slot_new branch that returns input.str(vm)? directly when cls is
str_type with no encoding. Subtype construction and the bytes-decoding
path are unchanged.

Unmasks test_str.StrTest.test_conversion (11 assertTypedEqual cases).