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)
```

Start by tracing PyCall's PyObject getindex path for the ax[:] call and the Julia 0.5/0.6 difference described here. Reproduce the shown PyPlot example and confirm that Colon indexing no longer raises MethodError.

Written by the indexing model from the issue text.