[Python] Set __spec__ attribute and not __cached__ for ROOT facade

The Python documentation [1] says:

> It is strongly recommended that you use module.__spec__.cached instead
  of module.__cached__.

So this commit suggests to transfer the full `__spec__` attribute from
the ROOT module to the facade, instead of `__cached__`.

This also avoids errors when importing ROOT with Python 3.15, where
`__cached__` will cease to be set or taken into consideration by the
import system or standard library.

Closes #21787.

[1] https://docs.python.org/3/reference/datamodel.html#module.__cached__

Author: guitargeek

bindings/pyroot/pythonizations/python/ROOT/_facade.py

@@ -142,7 +142,7 @@ def __init__(self, module, is_ipython):
         self.__all__ = module.__all__
         self.__name__ = module.__name__
         self.__file__ = module.__file__
-        self.__cached__ = module.__cached__
+        self.__spec__ = module.__spec__
         self.__path__ = module.__path__
         self.__doc__ = module.__doc__
         self.__package__ = module.__package__