Add support for `list[::1]` with Ruby 3.4+

[kou]

Ruby 3.4 rejected step against a begin-less range ((nil..nil).step(N)).
See also: https://bugs.ruby-lang.org/issues/18368

We need to consider how we can emulate Python's slice(None, None, -1) in Ruby >= 3.4.

See also: #191

[kou]

How about the followings?

(nil..nil).step(-1) # current style
(..nil).step(-1) # shorter version

PyCall::Slice.new(step: -1)
PyCall::Slice.step(-1)
PyCall.slice(step: -1)
PyCall.slice(nil, nil, -1)
PyCall.slice(..nil, -1)