Generate NumPy scalars for test inputs

[asmeurer]

We should update the arrays strategy so that it can generate NumPy scalars whenever it would generate a 0-D array. This can be done by doing something like

if a.ndim == 0 and draw(booleans()):
    a = a[()]

which would be a no-op in any other library.

There are several issues with NumPy scalars and the array API that are not being caught by the test suite because they aren't being tested. See for instance #287 (NumPy scalars do not have in-place methods), and data-apis/array-api-compat#165 (NumPy scalars were missing __array_namespace__, which to be fair isn't tested anyways #248).