PyAPI: use a new type for storing the deferred result of bpy.props

This is needed to support Python 3.10's Postponed annotation evaluation.

It also simplifies type checking.

source/blender/python/intern/bpy_props.h

@@ -30,6 +30,16 @@ PyObject *BPy_PointerProperty(PyObject *self, PyObject *args, PyObject *kw);
 PyObject *BPy_CollectionProperty(PyObject *self, PyObject *args, PyObject *kw);
 StructRNA *pointer_type_from_py(PyObject *value, const char *error_prefix);
 
+typedef struct {
+  PyObject_HEAD
+      /* This isn't GC tracked, it's a function from `bpy.props` so it's not going away. */
+      void *fn;
+  PyObject *kw;
+} BPy_PropDeferred;
+
+extern PyTypeObject bpy_prop_deferred_Type;
+#define BPy_PropDeferred_CheckTypeExact(v) (Py_TYPE(v) == &bpy_prop_deferred_Type)
+
 #define PYRNA_STACK_ARRAY RNA_STACK_ARRAY
 
 #ifdef __cplusplus