$ pip install cupy==8.0.0b4
...
    Modules:
      cuda      : Yes (version 11000)
      cusolver  : Yes
      cudnn     : No
        -> Include files not found: ['cudnn.h']
        -> Check your CFLAGS environment variable.
      nccl      : No
        -> Include files not found: ['nccl.h']
        -> Check your CFLAGS environment variable.
      nvtx      : Yes
      cutensor  : No
        -> Include files not found: ['cutensor.h']
        -> Check your CFLAGS environment variable.
      cub       : Yes
      thrust    : Yes (version 100909)
...
    NOTICE: Skipping cythonize as cupy/cuda/cub.pyx does not exist.
...
    RuntimeError: Missing file: cupy/cuda/cub.cpp
    Please install Cython 0.28.0 or later. Please also check the version of Cython.
    See https://docs-cupy.chainer.org/en/stable/install.html for details.