Only support NumPy 2.0 as a backend

[asmeurer]

Obviously not just yet, but once NumPy 2.0 is released and stable, we should drop support for NumPy 1.26 as a backend. That would allow us to remove all the compatibility code and only have code related to strictness.

The actual backend in array-api-strict doesn't really matter that much, but this would limit the usability for libraries that use NumPy internally in addition to the array API, so we should only do this once the ecosystem seems generally caught up on NumPy 2.0 support.

[asmeurer]

Also I'm not sure if it's worth trying to implement some of the new dlpack features without direct numpy support for them (I need to do some more investigation), so we may want to just always keep this package pinned to the latest numpy. See #35