Fix #75. Improve slice key type typing to be more specific.

[cmeyer]

Fixes #75.

If this isn't sufficient, please provide a line of client code that should pass typing but doesn't.

[Brow71189]

This fixes my issue, thanks! Tested with the following code snippet:

from nion.data import xdata_1_0 as xd
import numpy as np

xdata = xd.new_with_data(np.zeros((100, 100, 100)))

xd.data_slice(xdata, (slice(10, 20), slice(20, 40)))