pybind11::str::raw_str simplification (for Python 2)

For Python 2, convert directly to unicode (instead of converting to str first, followed by encoding).

Simpler code, and more obviously equivalent to `unicode(...)` in the interpreter.

Author: rwgk

include/pybind11/pytypes.h

@@ -930,11 +930,10 @@ class str : public object {
 private:
     /// Return string representation -- always returns a new reference, even if already a str
     static PyObject *raw_str(PyObject *op) {
-        PyObject *str_value = PyObject_Str(op);
-        if (!str_value) throw error_already_set();
 #if PY_MAJOR_VERSION < 3
-        PyObject *unicode = PyUnicode_FromEncodedObject(str_value, "utf-8", nullptr);
-        Py_XDECREF(str_value); str_value = unicode;
+        PyObject *str_value = PyObject_Unicode(op);
+#else
+        PyObject *str_value = PyObject_Str(op);
 #endif
         return str_value;
     }