Fix uvloop to 0.14.0

[ahopkins]

uvloop is dropping python 3.6 support. I think we should also, but I am not quite ready to do that in v21.3. In the mean time, let's fix uvloop to 0.14. Also making a PR for LTS.

[codecov]

Codecov Report

Merging #2024 (c942a5c) into master (ffca01f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2024   +/-   ##
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  Coverage   91.45%   91.45%           
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  Files          28       28           
  Lines        3171     3171           
  Branches      571      571           
=======================================
  Hits         2900     2900           
  Misses        192      192           
  Partials       79       79           

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[ashleysommer]

Why fix it to exactly 0.14? That could be an upper limit:
uvloop>=0.5.3,<=0.14.0
That might be more compatible for users who require uvloop pinned to other versions.

[ahopkins]

Fair enough. That is what I did for the two LTS branches: #2025, #2026