don't patch event loop for tornado 6.1+

[bollwyvl]

tornado 6.1 works with the default event loops on all platforms now. I've been working on rooting it out of other repos, still a couple more to do.

As "the future," and having an imminent major version bump, the jupyter_server PR requires tornado 6.1

• Remove pytest11 entrypoint and plugin, require tornado 6.1, remove asyncio patch, CI work jupyter-server/jupyter_server#339

As this package is already widely deployed, this uses the "just check the tornado version" approach:

• do not apply asyncio patch for tornado >=6.1 jupyter/notebook#5907
• add github actions, bail on asyncio patch for tornado 6.1 ipython/ipykernel#564

Would happily do that, here, if we don't mind pining to the "bleeding edge" tornado, and remove some cruft.

[codecov-io]

Codecov Report

Merging #142 (7b37be2) into master (52a9118) will decrease coverage by 0.37%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #142      +/-   ##
==========================================
- Coverage   74.81%   74.44%   -0.38%     
==========================================
  Files          21       21              
  Lines        1608     1608              
==========================================
- Hits         1203     1197       -6     
- Misses        405      411       +6     

Impacted Files	Coverage Δ
jupyterlab_server/tests/utils.py	45.65% <100.00%> (-13.05%)	⬇️

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[blink1073]

Thanks!