[BUG] NumPy emits ImportWarning when importing numpy.random on PyPy3.9 wh

[mattip]

Describe the bug

$ pypy3.9 -Werror -c "import numpy"
numpy/__init__.py:152: in <module>
    from . import random
numpy/random/__init__.py:180: in <module>
    from . import _pickle
numpy/random/_pickle.py:1: in <module>
    from .mtrand import RandomState
numpy/random/mtrand.pyx:19: in init numpy.random.mtrand
    ???
E   ImportWarning: can't resolve package from __spec__ or __package__, falling back on __name__ and __path__

The code imports another cython c-extension module from inside a cython c-extension module.

Code to reproduce the behaviour:

The exception points to this

from ._mt19937 import MT19937 as _MT19937

Expected behaviour

Without -Werror the import works, so it is "just" the warning that is problematic.

OS

Ubuntu 22.04

Python version

pypy3.10 (pre-release), pypy3.9

Cython version

from 0.29.31 to HEAD

Additional context

Maybe need to extend #4764 to more cases? I think this might have to do with the CYTHON_PEP489_MULTI_PHASE_INIT=0 for PyPy. If I define it to 1 the warning goes away and NumPy tests pass. I couldn't easily grep for an existing test for importing another module within a module, but there must be one.

[da-woods]

If CYTHON_PEP489_MULTI_PHASE_INIT now works on PyPy then we should enable it.

(Not sure of the rest of the issue)