Remove obsoleted asyncio-patch fixture

[kevin-bates]

With the minimum version bump to Tornado 6.1, we should no longer be using the patch in our fixtures. This update removes that fixture, as well as the update to the io_loop fixture needed to enable the patch.

[codecov-io]

Codecov Report

Merging #412 (0017061) into master (3220f4f) will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #412      +/-   ##
==========================================
- Coverage   69.90%   69.84%   -0.07%     
==========================================
  Files          57       57              
  Lines        6320     6310      -10     
  Branches      847      847              
==========================================
- Hits         4418     4407      -11     
- Misses       1599     1600       +1     
  Partials      303      303              

Impacted Files	Coverage Δ
jupyter_server/pytest_plugin.py	91.62% <ø> (-0.45%)	⬇️
jupyter_server/serverapp.py	64.57% <0.00%> (-0.12%)	⬇️

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[blink1073]

Thanks!