Fix GDString.to_pystr

Author: touilleMan

src/godot/builtins_pxd/class.pxd.j2

@@ -30,7 +30,7 @@ cdef class {{ spec.name }}:
         __{{ spec.name }}_constructor_0(&ret._gd_data, NULL)
         return ret
 {% if spec.name == "GDString" %}
-    cdef inline gd_string_t to_pystr(self):
+    cdef inline object to_pystr(self):
         return gdstring_to_pystr(&self._gd_data)
 {% endif %}
 {% for m in spec.members %}

src/godot/builtins_pyx/class.pyx.j2

@@ -35,7 +35,7 @@ cdef class {{ spec.name }}:
         return "<{{ spec.name }}>"
 {% if spec.name == "GDString" %}
     def __str__(self):
-        self.to_pystr()
+        return self.to_pystr()
 {% endif %}
 {% if spec.constants %}
     # Constants