`test_linear_interpolation` is failing for NumPy 2.x

[jcannon-gplates]

For #114 to succeed, the reason for the test_linear_interpolation failing needs investigation.

The error happened after adding support for NumPy 2.x on PyPI (in 7cfce70).

Once that error is sorted out then new 2.3.4 wheels (that support both NumPy 1.x and 2.x) can get uploaded to PyPI.

[brmather]

This appears to be resolved once stripy is actually built against NumPy 2.x (the fix for #114, on branch numpy2-build-robustness).

I built that branch locally against numpy 2.5.1 and ran the full suite under numpy 2.5.1 — all 46 tests pass, including both test_linear_interpolation variants ([False] and [True]) for sTriangulation and Triangulation:

46 passed, 2 warnings

So the earlier failure looks like a build-time artefact rather than a numerical regression — most likely a mix of a NumPy-1-built extension being run under NumPy 2 (the core #114 problem), compounded by the test workflow running pytest from the repo root, which imported the ./stripy source tree (no compiled .so) instead of the installed package. Both are fixed on the branch (build against numpy≥2; CI.yml now runs pytest from a neutral directory, plus a py3.9–3.13 × numpy{<2,≥2} matrix that exercises exactly this path).

Suggest we close this once the branch is merged and CI is green.