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
...
```