getindex(::PyObject, ::Union{Symbol, AbstractString}) should not call getproperty

Sometimes the Python object that underlies your PyObject needs to be accessed with a slice-like notation with a string rather than dot notation. `dict`s would be one example, but since they're converted to Julia `Dict`s, they don't really highlight the issue. Here is where I'm running into it. Unfortunately I don't have a better example than this (this one requires having MATLAB installed to run):
```julia
julia> using PyCall
```
```julia
julia> py"""
import matlab.engine
eng = matlab.engine.start_matlab()
"""
```
```julia
julia> eng = py"eng"
PyObject
```
```julia
julia> py"""
eng.workspace['a'] = 0.0
"""
```
```julia
julia> eng.workspace
Name Size Bytes Class Attributes

a 1x1 8 double

PyObject
```
```julia
julia> eng.workspace["a"]
ERROR: KeyError: key "a" not found
...
```

Start by locating the getindex(::PyObject, ::Union{Symbol, AbstractString}) implementation and reproduce the reported eng.workspace["a"] failure if MATLAB is available. Done means string indexing accesses the underlying Python object correctly instead of following getproperty and no longer raises KeyError for the shown example.

Scritto dal modello di indicizzazione a partire dal testo della issue.