matplotlib outputs text representation instead of figure

# Provide a minimally reproducible example

[nbdev-matplotlib-test](https://github.com/idahlke-blacksky/nbdev-matplotlib-test)
[Pages](https://idahlke-blacksky.github.io/nbdev-matplotlib-test)

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 (). 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.

Start by running the linked nbdev-matplotlib-test notebooks with nbdev_docs using different worker counts, then inspect the nbdev_docs entry point and how notebook outputs are collected. Done means every matplotlib figure renders as a figure rather than a text representation, including when the number of figure-containing notebooks exceeds the worker count.

Written by the indexing model from the issue text.