Allow to use NumPy 2.

[vnmabus]

Describe the proposed changes

All tests are now executed with NumPy 2. This is important, because the representation of arrays (and thus the expected doctest output) has been changed in this version.

Checklist before requesting a review

• I have performed a self-review of my code
• The code conforms to the style used in this package
• The code is fully documented and typed (type-checked with Mypy)
• I have added thorough tests for the new/changed functionality