Bump numpy build version to `>=1.25` after Python 3.8 deprecation

[timkpaine]

xref: https://github.com/Point72/csp/pull/126#issue-2163667290

According to https://numpy.org/doc/stable/dev/depending_on_numpy.html#build-time-dependency,
we can build against numpy 1.25 or newer and numpy 1.19 or newer should be able to import the build.
However, numpy 1.25 isn't supported on python 3.8, so I've decided to set a lower bound on an older
version of numpy on the build host, which should also avoid ABI mismatch errors.