[pythoncall] jl.Pair generates tuple instead of Julia pair

**Affects:** JuliaCall

**Describe the bug**
In python, with
`from juliacall import Main as jl`
`jl.Pair(1,2)`
it will generate a tuple instead of julia pair like `1=>2`.

**system info**
Please provide detailed information about your system:
- Mac Ventura 13.2.1
- The version of Julia 1.8.5, Python 3.9.7, JuliaCall 0.9.12

Start by reproducing `jl.Pair(1,2)` through `from juliacall import Main as jl` with the reported Julia, Python, and JuliaCall versions. Trace the binding behind `jl.Pair` and compare its result with the expected Julia pair `1=>2`; done means the call produces a Julia pair rather than a tuple.

索引モデルが issue の本文から書いたものです。