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`?