I wrote a new different API: https://github.com/python/cpython/pull/780

New C API for variables only initialized once to be able to clear
them at exit:

    New macro _Py_ONCEVAR(var) to declare a variable
    New macro _PY_ONCEVAR_INIT(var, expr) to initialize a variable
    once
    New function _PyOnceVar_Set() to explicitly set a variable once
    to initialize it
    New _PyOnceVar_Fini() function clearing all variables (initialized
    once) at exit

Variables keep their PyObject* type, but the API hides an internal C array tracking all Python objects to Py_DECREF them at exit in _PyOnceVar_Fini().

I used Nick's naming scheme since it seems like pthread has an existing API which is similar, but different (my API doesn't require an initialization callback).

I really prefer the second API using PyObject*.