Don't try to re-setup any GUI event loop (for PyJulia)

[tkf]

If matplotlib.pyplot exists, it means that matplotlib backend is already configured. It may mean that PyPlot.jl is called via PyJulia. Don't setup any GUI event loop in this case since matplotlib does not support switching GUI backends.

With this patch, %julia magic can show interactive notebook plots:

[tkf]

Maybe it's better to not switch only from "unknown" backend (e.g., nbagg)?

    if haskey(matplotlib, :pyplot)
        # If matplotlib.pyplot exists, it means that matplotlib
        # backend is already configured.  It may mean that PyPlot.jl
        # is called via PyJulia.
        backend = lowercase(matplotlib[:get_backend]())
        if !haskey(matplotlib2gui, backend)
            return (backend, :none)
        end
    end