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.

首先查看链接的 Pluto.jl issue #511，并在受影响的 Mac 安装环境中重现后端选择问题。比较自动选择的后端与手动配置的后端，包括 notebook 上下文中的 qt5agg。当 PyPlot 能够在 Pluto 中无需手动配置即可正确选择可用后端，并且已了解受影响的情况时，即视为完成。

由索引模型根据 Issue 内容生成。