ipykernel 5.3.3. compatibility issue with qt

Hi,

Taking the advised of @blink1073, I'm here reporting the issue found in ipykernel 5.3.3.

Basically, my qt (run in jupyter notebook) doesn't work any more after I upgraded ipykernel 5.3.3. In my testing, downgrading to 5.3.0 or 5.3.2 solves the problem, perhaps there's some incompatibility in this 5.3.3 version.

Here is what I got:

Test this code in jupyter notebook:
```
%matplotlib qt
import matplotlib.pyplot as plt
x = [1,2,3]
y = [5,4,5]
plt.plot(x,y)
```

Before upgrade to ipykernel==5.3.3

After:

_Originally posted by @jacky4eyes in https://github.com/ipython/ipykernel/pull/525#issuecomment-660470221_

(My system is Win10 - 10.0.18363)