Bump meson
Meson 1.4.0 adds first class support for NumPy > 2.0 detection https://mesonbuild.com/Release-notes-for-1-4-0.html#new-numpy-custom-dependency

Not an urgent upgrade but figured worth starting
I believe the pypy failure is a Cython issue - see https://github.com/cython/cython/issues/6099

Generally I was surprised that the amount of warnings changed with this PR. But I fixed a few / ignored others (mostly for MSVC). Would be nice to get a lot of the MSVC warnings fixed up but I think that will take a dedicated initiative.
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