asyncio.sleep hangs with matplotlib qt5

This cell will not finish
``` python
%matplotlib qt5
import asyncio
await asyncio.sleep(1)
print("done")
```
Except if the other cell is run:
``` python
import matplotlib.pyplot as plt
plt.figure()
```

Reproduce the two notebook cells shown in the issue, first with the Qt5 backend and then after creating a figure. Trace the interaction between asyncio.sleep and the matplotlib Qt5 integration; done means the first cell completes and prints "done" without requiring the second cell.

Written by the indexing model from the issue text.