Matplotlib's plt.show() does not work

[nvaytet]

See #595 (comment)

In a script or ipython console:

import matplotlib.pyplot as plt
import scipp as sc
f = sc.arange("x", 5.0).plot()
plt.show()

shows nothing, while using f.show() will show the figure (note that the figure may dissapear instantly again inside a script).

[nvaytet]

I am quite sure it's something to do with how we create the figure: https://github.com/scipp/plopp/blob/main/src/plopp/backends/matplotlib/utils.py#L79-L83

Replacing this code with a simple return plt.figure() seems to fix the issue, but creates new ones, such as the one in the comment where the figures start auto-showing in the notebooks, when we actually want them to only show when the repr of the figure is requested.