Pin to NumPy <3 (#658)

This change is as discussed between Jamie, Fabian and me; and
subsequently posted publicly in
#643 (comment).
Pinning to NumPy <= 2.3 would also be an ideal option, but we have opted
for a more relaxed pin to NumPy <3, owing to the fact that we have
limited availability to add new features and hence fewer chances of
anything major breaking.

This change is being merged after #643, and has been split from it
because it is not directly related to the proposition of adding tests
against nightly dependencies.

pyproject.toml

@@ -44,7 +44,7 @@ keywords = [
   "SciPy",
 ]
 dependencies = [
-  "numpy",
+  "numpy<3",
 ]
 # dynamic = ["version"]