import cupy 9.0.0a2 doesn't work on Windows 10

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If it is a bug report, please include the following information:

• Conditions (you can just paste the output of python -c 'import cupy; cupy.show_config()')
  • CuPy version 9.0.0a2
  • OS/Platform: Windows 10
  • CUDA version: 10.2
  • cuDNN/NCCL version (if applicable)
• Code to reproduce

>>> import cupy

• Error messages, stack traces, or logs

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\Amin\anaconda3\envs\py38\lib\site-packages\cupy\__init__.py", line 57, in <module>
    from cupy import fft  # NOQA
  File "C:\Users\Amin\anaconda3\envs\py38\lib\site-packages\cupy\fft\__init__.py", line 1, in <module>
    from cupy.fft._fft import fft  # NOQA
  File "C:\Users\Amin\anaconda3\envs\py38\lib\site-packages\cupy\fft\_fft.py", line 9, in <module>
    from cupy.fft import config
  File "C:\Users\Amin\anaconda3\envs\py38\lib\site-packages\cupy\fft\config.py", line 13, in <module>
    from cupy.fft._callback import get_current_callback_manager  # NOQA
  File "cupy\fft\_callback.pyx", line 35, in init cupy.fft._callback
AttributeError: 'NoneType' object has no attribute 'split'

[kmaehashi]

we’ll soon make a release including this fix as v9.0.0a3.

Unfortunately, as many test failures are observed in #4362, we decided not to release v9.0.0a3 this time and will work on fixing the Windows issues towards v9.0.0b1 planned next month. We are going to take down v9.0.0a2 Windows wheels. We’re sorry for the inconvenience.

[kmaehashi]

Thank you for the feedback, we’ll soon make a release including this fix as v9.0.0a3.