But let's make str() of a slice more suggestive of actual slicing, and
as a bonus, make slices easier to create too.

    str(slice(None, None, None)) => "slice[:]"

following all the later discussion, I'd like to come back to this initial part of this thread:
An even briefer for for just "str" would be to omit the 'slice' part

    str(slice(None, None, None)) => "[:]"
etc.  Since it has colons in there, it's clear it is a slice.  And it is very brief notation.