PyPlot on Mac with Pluto - backend identification problems.

Hi, I just cross-link this issue from Pluto here: https://github.com/fonsp/Pluto.jl/issues/511
PyPlot seems to work smoothly in Pluto, but on some (?) Mac installations it seems
- to either figure out the wrong backend
- or that somehow the qt5agg backend doesn't see that it runs from a notebook

Manually setting the backend seems to help.

Start by reviewing the linked Pluto.jl issue #511 and reproducing the backend-selection problem on an affected Mac installation. Compare the automatically selected backend with the manually configured backend, including qt5agg in a notebook context. Done means PyPlot selects a working backend correctly in Pluto without manual configuration, with the affected cases understood.

Written by the indexing model from the issue text.