Prevent importing chainer immediately raises an error with cupy>=10

[belltailjp]

When cupy>=10 is installed, importing chainer immediately raises the following error.

>>> import cupy; cupy.__version__
'10.0.0'
>>> import chainer
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.9/site-packages/chainer/__init__.py", line 10, in <module>
    from chainer import backends  # NOQA
  File "/usr/local/lib/python3.9/site-packages/chainer/backends/__init__.py", line 1, in <module>
    from chainer.backends import cuda  # NOQA
  File "/usr/local/lib/python3.9/site-packages/chainer/backends/cuda.py", line 124, in <module>
    _cupy_major = numpy.lib.NumpyVersion(cupy.__version__).major
  File "/usr/local/lib/python3.9/site-packages/numpy/lib/_version.py", line 59, in __init__
    raise ValueError("Not a valid numpy version string")
ValueError: Not a valid numpy version string

This PR is to fix this issue.
I am quite aware that there is (and should be) absolutely no guarantee that chainer and the latest cupy work fine together. But for those who have to use or maintain codes that still rely on some of chainer functionalities with their own risk, I'd like to prevent the situation where just importing chainer immediately fails.

[kmaehashi]

Thanks!

[kmaehashi]

Btw, you can also workaround the issue by upgrading to NumPy 1.21 or later.
numpy/numpy#19214

[kmaehashi]

/test

[kmaehashi]

Confirmed locally. Thanks!