Fix compatibility with Ruby 3.4.0 in ranges

[annzenkina]

CI in the upstream fails for the latest Ruby version 3.4:

  1) PyCall::PyObjectWrapper#[] when the given index is an Enumerable that is created by Range#step is expected to eq [2, 4, 6, 8]
     Failure/Error: expect(list[(nil..nil).step(-1)]).to eq(PyCall::List.new([*1..10].reverse))

     ArgumentError:
       #step for non-numeric beginless ranges is meaningless
     # ./spec/pycall/pyobject_wrapper_spec.rb:164:in 'Range#step'
     # ./spec/pycall/pyobject_wrapper_spec.rb:164:in 'block (4 levels) in <module:PyCall>'

It looks like this happens due to this change in the most recent Ruby version which doesn't allow to use "beginless" ranges anymore.

This PR sets the range explicitly so we can avoid failing with an exception