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Is `cupy.cuda.runtime` set to None too early during interpreter shutdown?

See original GitHub issue

Conditions (you can just paste the output of python -c 'import cupy; cupy.show_config()')

$ python -c 'import cupy; cupy.show_config()'
CuPy Version          : 7.0.0rc1
CUDA Root             : /usr/local/cuda
CUDA Build Version    : 10000
CUDA Driver Version   : 10000
CUDA Runtime Version  : 10000
cuDNN Build Version   : 7604
cuDNN Version         : 7604
NCCL Build Version    : None
NCCL Runtime Version  : None

Tested with a fresh build on the latest master in a fresh conda environment.

Code to reproduce

>>> import cupy as cp
>>> stream = cp.cuda.Stream()
>>> # ...then exit the interpreter
>>>
Exception ignored in: <bound method Stream.__del__ of <cupy.cuda.stream.Stream object at 0x7f75830870b8>>
Traceback (most recent call last):
  File "cupy/cuda/stream.pyx", line 186, in cupy.cuda.stream.Stream.__del__
AttributeError: 'NoneType' object has no attribute 'streamDestroy'

I don’t think I’ve seen this error before. Looks like runtime is set to None when stream.__del__() is invoked, but this is unexpected because, according to PEP 442, starting at Python 3.4 the modules unloaded during interpreter shutdown are no longer set to None, and I’m on Python 3.6.7.

Can this be reproduced? Thanks.