spec: 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

See also:

• https://bugs.ruby-lang.org/issues/18368
• Make Range#step to consistently use + for iteration  ruby/ruby#7444

[kou]

Let's work on list[::1] support with Ruby 3.4+ in #206.

Let's fix CI in this PR.

Let's use cc9a073 by @kojix2 in #197.