Update matplotlib backend

[ptsavol]

Our matplotlib is using a legacy backend. While these still work, they should be updated.

https://matplotlib.org/stable/users/explain/figure/backends.html

These should be updated:

helpers.py matplotlib.use("Qt5Agg") -> matplotlib.use("qtagg")

plot_canvas.py from matplotlib.backends.backend_qt5agg import FigureCanvasQTAgg -> from matplotlib.backends.backend_qtagg import FigureCanvasQTAgg

plot_widget.py from matplotlib.backends.backend_qt5agg import NavigationToolbar2QT as NavigationToolBar -> from matplotlib.backends.backend_qtagg import NavigationToolbar2QT as NavigationToolBar