I've tried running this code in Python 3.6:

from _struct import Struct
for i in range(100000):
    L = [Struct.__new__(Struct) for j in range(1000)]
    for s in L:
        try:
            x = s.pack_into(bytearray())
        except SystemError:
            pass

I've run it 6 times, for a total of 600 million calls to Struct.__new__ 
and pack_into, and I cannot reproduce any crash or segfault. An 
exception (SystemError) is the correct behaviour.

Is anyone able to try it under Python 3.7?

Unless somebody is able to demonstrate a segfault or core dump, or 
otherwise demonstrate a problem with the C code, I think this ticket 
ought to be closed.