Remove upper bound for NumPy

[janjagusch]

Closes #432.

Motivation

This package is currently not compatible with numpy>=2, which was released this summer (see here). Because of that, this package is incompatible with those that have already dropped numpy<2 support.

Removing the numpy upper bound should not break anything, as this package doesn't actually make direct use of it (only indirectly via, e.g., pandas or in its testing suite).

For more details on the problem with upper version constraints, see:

• https://iscinumpy.dev/post/bound-version-constraints/
• https://iscinumpy.dev/post/poetry-versions/

Changes

This PR removes the upper bound for numpy. It also adds a testing matrix to ensure that the unit tests are run against both numpy=1.* and numpy=2.*.