Describe the issue:

A test that invokes C APIs segfaults when run from multiple threads in parallel in python 3.13t:

The test invokes a test-specific C function, which however upon cursory inspection doesn't seem to have anything untowards in it. The C function in turn calls: Py_SET_TYPE, PyArray_RegisterDataType, PyArray_DescrFromType:

Reproduce the code example:

1. deploy numpy through https://github.com/rgommers/pixi-dev-scipystack/
2. apply patch (see DEV: stop monkeypatching and use of autouse=True fixtures by default in test suite #29090):

--- a/numpy/conftest.py
+++ b/numpy/conftest.py
@@ -147,9 +147,10 @@ def check_fpu_mode(request):
 def add_np(doctest_namespace):
     doctest_namespace['np'] = numpy
 
+
 @pytest.fixture(autouse=True)
-def env_setup(monkeypatch):
-    monkeypatch.setenv('PYTHONHASHSEED', '0')
+def env_setup():
+    os.environ['PYTHONHASHSEED'] = '0'

3. pixi run test-nogil -- -k test_custom_structured_dtype --parallel-threads=32

Python and NumPy Versions:

python 3.13t
numpy git tip 2025-06-04

Runtime Environment:

32-core x86_64 Linux host