Missing Colon method

This code used to work in 0.5, but fails in 0.6. I'm assuming that this is PyCall's responsibility, and not PyPlot's.
```julia
julia> using PyPlot
Fontconfig warning: line 146: blank doesn't take any effect anymore. please remove it from your fonts.conf

julia> fig, ax = plt[:subplots](2, 2)
PyObject (, array([[,
],
[,
]], dtype=object))

julia> ax[:]
ERROR: MethodError: no method matching getindex(::PyCall.PyObject, ::Colon)
```

首先跟踪 PyCall 中 PyObject 针对 ax[:] 调用的 getindex 路径，以及此处描述的 Julia 0.5/0.6 差异。重现所示的 PyPlot 示例，并确认 Colon 索引不再引发 MethodError。

由索引模型根据 Issue 内容生成。