This is another catch of using PyType_FromSpec (similar to  https://bugs.python.org/issue26979 but also completely different).

Because PyType_FromSpec produces a heap type it gets the Py_TPFLAGS_HEAPTYPE flag set on the newly produced type.  To enforce the immutability of built-in types type_setattro checks this flag.  Therefore these types end up being mutable, e.g:

import _ssl
_ssl.SSLError.x = 42

In addition to not providing parity for replacing PyType_Ready in the stable API, it also means that this could presumably bleed across sub-interpreters or just provide surprising and bad results.