numpy pinned to <2 in pyproject.toml

Assuming PR #49 is merged, the only pinning we're doing in `pyproject.toml` is `numpy<2`. A PR that reverses the pinning (`numpy>=2`) along with the test data regenerated for the new pinning should be the way forward I think. (numpy 1.x lasted 15 years, so there's some long-term value in pinning to 2 and above).