Just so you know, when we look at the changes to CPython, the easiest fix is to add these lines in cffi:

    #if PY_VERSION_HEX >= 0x03080000
    # define Py_BUILD_CORE
    #  include "internal/pycore_pystate.h"
    # undef Py_BUILD_CORE
    #endif

But if we're looking for a cleaner way to fix this, then cffi's needs could be covered by adding a general-purpose-storage dict to PyInterpreterState (like PyThreadState->dict, but per sub-interpreter instead of per thread), and an API function to get it.