Update CuPy base version

[LWisteria]

Currently based on v2.1.0.

We plan to update the version to v4.x or v5.x or more.

[nsakabe-fixstars]

Summaries of difference between (CuPy) versions:
https://docs-cupy.chainer.org/en/stable/upgrade.html

[LWisteria]

I tried to upgrade the base version to estimate how difficult it is.

• v2.1.0 -> v2.1.0.1 is very easy
• v2.1.0.1 -> v2.2.0 is a little difficult but not so much. It takes only one hours.

The key points are

• Many conflicts are caused by renaming cupy to clpy
• Even if there are no conflicts, the new commits on CuPy may include "cupy" keywords. They must be checked carefully (CI would tell us)
• _version.py causes conflict every time... why?

[LWisteria]

Note that the above experimental branches would not work well, I've never checked to build and run.

[LWisteria]

The easiness shown above is from my commits made for the first time as ClPy.

In anticipation of the base version update, I firstly renamed files by git mv (f5ff2ee, 58aa4ed, and 664ccee) before overwriting ClPy changes (dec25c3) to follow later changes.