PyAPI: add read-only 'is_valid' attribute to mathutils types

There was no convenient way to check if the owner
of a mathutils type was valid.

Added to support issue reported in T91111.

source/blender/python/mathutils/mathutils_Vector.c

@@ -2551,6 +2551,11 @@ static PyGetSetDef Vector_getseters[] = {
      (setter)NULL,
      BaseMathObject_is_frozen_doc,
      NULL},
+    {"is_valid",
+     (getter)BaseMathObject_is_valid_get,
+     (setter)NULL,
+     BaseMathObject_is_valid_doc,
+     NULL},
     {"owner", (getter)BaseMathObject_owner_get, (setter)NULL, BaseMathObject_owner_doc, NULL},
 
     /* Auto-generated swizzle attributes, see Python script above. */