Numpy version policy -> test matrix

[shumpohl]

Numpy just released 2.0.0 the first major release since 2006. I think a reasonable goal is to support all numpy versions that have not reached EOL. Currently that is 2.0, and >=1.24. This means we should setup a testing matrix with numpy 1 and numpy 2. From the migration guide I gather that the only problem we can run into are different numeric precision propagation behaviour.

https://numpy.org/doc/stable/release/2.0.0-notes.html

https://numpy.org/doc/stable/numpy_2_0_migration_guide.html#numpy-2-migration-guide

[shumpohl]

Implemented in #859