matplotlib outputs text representation instead of figure

[idahlke-blacksky]

Provide a minimally reproducible example

nbdev-matplotlib-test
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I set up this experiment to reproduce the bug in a shareable environment. The notebooks all have the same plotting code. Note that some of the notebooks show a text representation instead of the rendered figure (<Figure size 640x480 with 1 Axes>). This happens for the last N notebooks anytime the number of notebooks with matplotlib figures exceeds the number of workers passed to nbdev_docs by N. I can locally run nbdev_docs n_workers 1 and force all notebooks except the first to fail. Increasing the number of workers decreases the number of failed notebooks expectedly.

Locally I am setting up with the conda env file, but the problem repeats on github without it.