How to set or modify tp_flags?

[fsh]

In an extension type written with py_class!(), is there a way to set or fix the tp_flags of the resulting PyTypeObject?

For this specific use-case I want to add Py_TPFLAGS_BASETYPE so the extension class can be sub-classed in Python. Is this a quote-unquote "difficult" thing to do while still using the py_class! macro?

Any example or code appreciated.