PyConfig is for configuring the runtime during initialization, not for storing runtime state. Currently insys.set_int_max_str_digits() we are modifying the interpreter's PyConfig directly.

That field should never be modified outside runtime init. Instead, the config value should be copied to a field onPyInterpreterState and that is what should be modified bysys (and used in longobject.c).

In fact, this is mostly what we were doing until a couple weeks ago (and what we still are doing in 3.11 and earlier). This was changed on main (3.12) ingh-96944. The fix is relatively trivial.

CC@gpshead