Incompatibility with numpy 2.2

[rileyjmurray]

In numpy 2.2, a deprecation expired regarding truthiness checks of empty arrays (e.g., bool(np.array([]))). Apparently this check happens often in the Labels class. For some reason pytest complains that the check happens here (even though it isn't clear why the problematic check would be happening here):

pyGSTi/pygsti/baseobjs/label.py

Line 130 in f144607

if state_space_labels is None or state_space_labels in ((), (None,)):

I don't have specific error messages to share since I just downgraded to numpy 2.1, but you can easily see the errors if you run the unit tests with numpy 2.2 installed.