Python does not print long long integer correctly when calling a C library using ctypes

I want to do some Python multithreading and bypass the GIL using ctypes.

I wrote some C code, test.c:

long long sum_range(long high)
{
  long i;
  long long s = 0LL;
 
  for (i = 0L; i < high; i++)
      s += (long long)i;

  return s;
}

Compile and link using

gcc -O3 -g -fPIC -c -o test.o test.c
ld -shared -o libtest.so test.o

Call it from a Python 3 script:

import ctypes
import os

testlib = ctypes.cdll.LoadLibrary(os.getcwd()+"/libtest.so")

sum_range = testlib.sum_range
sum_range.argtypes = [ctypes.c_long]
sum_range.restypes = ctypes.c_longlong

iterations=100000

print("sum_range(iterations) = {}".format(sum_range(iterations)))

This gives

sum_range(iterations) = 704982704

Instead of 4999950000. What is wrong here?

PS Adding a printf to the C code shows that the correct value of 4999950000 is being returned to the Python script.

PPS The correct value is printed when iterations is e.g. ten times smaller, so 10000.

PPPS A similar question was raised earlier but setting restypes apparently does not fix the problem.