Add _BaseSegment to simplify type annotations.

[udifuchs]

_BaseSegment is a private type alias for either PathSegment or Move classes.

It simplifies the type annotations and resolves most of the typing issue.

This commit also fixes the typing issues for slices (as in getitem).

Adding typing for slices make the behaviour of the Path class a bit more official.
Path behaves here a bit differently from most mutable sequences.
For example, a slice of an array is an array:

>>> from array import array
>>> a = array("i", [1, 2, 3])
>>> a[1:2]
array('i', [2])

Therefore it would make sense that a slice of a Path would also be a Path
and not a list.

We could make any usage of slice raise a NotImplementedError.
It would brake backward compatibility, but I would be surprised if anyone
is using slices of Path.

The other option is to apply this PR as is.