- replaced np.AxisError with np.exceptions.AxisError
- the `numpy.core` submodule has been renamed to `numpy._core`
- some parts of `numpy.core` have been moved to `numpy.lib.array_utils`

Except for `AxisError`, the updated imports are conditional on
the version of numpy, so the imports should work for numpy >= 1.26.

The conditional imports have been added to `npy_2_compat.py`, so the
imports elsewhere are unconditonal.