MNT: Consolidate NumPy/SciPy baseline source location

Noticed that it is duplicated here https://github.com/cupy/cupy/blob/d4a8e24cf685963f1c36be7b9cb7984a8676870b/cupy/testing/_helper.py#L27-L28 and here https://github.com/cupy/cupy/blob/bdc4a0f31ef072579c3b1cf690ee5ff5824b9523/docs/source/install.rst?plain=1#L26 whereas the source of truth is here (I think!) https://github.com/cupy/cupy/blob/bdc4a0f31ef072579c3b1cf690ee5ff5824b9523/docs/requirements.txt#L6-L8 Let's figure out a way to consolidate them in one place.