wxpython EventLoop -> GUIEventLoop

PyPlot doesn't work with wxpython on macOS. (I tried on julia 1.5.2 & latest julia from github).

output:
```julia-repl
julia> using PyPlot

julia> plot([1, 2])
ERROR: sys:1: wxPyDeprecationWarning: Using deprecated class EventLoop. Use GUIEventLoop instead.

# here goes infinite loop
```

the same using Plots with `pyplot()`:
```julia-repl
julia> using Plots

julia> pyplot()
Plots.PyPlotBackend()

julia> plot([1, 2])
ERROR: sys:1: wxPyDeprecationWarning: Using deprecated class EventLoop. Use GUIEventLoop instead.
```

I assume the problem is rather with `PyCall` as `EventLoop ` is called from there. Since I'm not very familiar with `wxpython` and just changing `EventLoop ` to `GUIEventLoop ` does not work, I had to open this issue here. Should I open it in `PyCall`?

Reproduce the failure with the PyPlot and Plots commands shown in the issue on macOS, then trace where PyPlot or PyCall invokes wxPython's EventLoop. Determine whether the fix belongs in PyPlot.jl or PyCall; done means plotting no longer emits the deprecation warning or enters an infinite loop.

Vom Indexierungsmodell aus dem Issue-Text verfasst.