Dropping Python 3.10

Currently CuPy still supports Python 3.10. However Python 3.10 goes EOL in October 2026. So next month. Given this, it is worth discussing when the last Python 3.10 release of CuPy should be. Also when we would like to remove it.

Should add Python 3.15 is being released around the time Python 3.10 will be EOL. So this provides an opportunity for this cleanup.

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Refs:

• https://github.com/cupy/cupy/issues/10259