I prefer Josh's wording.  The important point to me is that

>>> [1, 2][2:0] = "AB"
[1, 2, "A", "B"]

not an error or ["B", "A"] == [1, 2][2:0:-1].  I think too much talk about the endpoints obscures this important fact.  (I think I'd like it to be an error, since the interpretation of s[2:0] = t could reasonably be any of s[0:0] = t, s[1:1] = t, or s[2:2] = t, but I haven't thought carefully enough yet, and "backward compatibility".)

Note: Josh's wording is already used in 3.7 (https://docs.python.org/dev/library/stdtypes.html#common-sequence-operations, as of the timestamp of this message).  I didn't check if it's been backported.