Add numpy as a build requirement

[ma3ke]

To prevent an incompatibility with the changes to numpy integers in numpy 2.0 (i.e., cnp.int_t does not exist anymore; we could just use int there, as far as I understand), we must pin the numpy version for the build requirements to 1.26. (For now.)

The install_requirements (such as MDAnalysis) lead to the numpy install requirement being interpreted as ~1.26, so on the runtime end things are fine. The problem may have been screened away by build tests on systems that have a system-wide numpy<2.0 installation.

[ma3ke]

These changes are required to make things work on my system.

For clarity, it arose in my situation of:

• no system-wide installation of the numpy lib (broke the compilation at the #include for numpy),
• a system-wide installation of numpy@2.0.0 (broke on the presence of the obsolete? cnp.int_t), and
• installing into a fresh virtual environment (for isolation of context).

This does mean that I may have just moved another problem to the foreground on other systems, so some testing of this PR with a diverse set of machines may be warranted.

Have a good night ;)

[BartBruininks]

I agree, we should at some point take a look at numpy2.0 when MDAnalysis actually makes use of it as well.