TYP: Axis-related annotations
I tried to limit `_typing.AxisInt` to `Literal[0, 1]`. That might be feasible in the future when mypy supports more literal math (e.g., `1 - Literal[0, 1] = Literal[0, 1]`), but would cause too many errors for now.

This PR adds a few Axis-related annotations but does not restrict `AxisInt` to `{0, 1}`.
Thanks @twoertwein 