Deprecate AxisError, itemgetter, methodcaller

[crusaderky]

These classes and functions no longer have a reason to exist:

• dask.array.utils.AxisError (backwards compatibility around numpy.AxisError - no longer needed with the current minimum numpy dependency)
• dask.utils.itemgetter (serializable variant of operator.itemgetter - which can be serialized as of Python 3.7)
• dask.utils.methodcaller (serializable variant of operator.methodcaller - which can be serialized as of Python 3.7)

They are undocumented and as such they should be treated as internals. However it has been agreed at the maintainers meeting that we should go through a deprecation cycle for peace of mind.

[crusaderky]

Correction: dask.utils.methodcaller is not the same as operator.methodcaller, as the former allows passing arguments to the method at call time, while the latter forces you to pass them at build time.