I need to use tornado 4.5.3 because of this issue jupyter/notebook#3397
I tried to connect to an ipycluster and get the following error:
"RuntimeError: IOLoop is already running"
This problem was introduced quite recently because it was not present in 6.1.1.
Actually, I also tried installing ipython/ipykernel#323 and ipython/ipython#11155 with the new tornado, that that didn't help.