check for already set asyncio event loop policy

[syfluqs]

For projects using app.create_server instead of app.run, if event loop policy is already set to uvloops policy externally, sanic will reset it and may cause two asyncio event loops to spawn, one from the earlier policy and one with the policy set by sanic.

This patch will check if asyncio event loop policy is already set as uvloop policy and won't reset it, if so.

[ahopkins]

Looks okay. Just take a look at the linting issue and fix.

sanic/server.py:32:1: W293 blank line contains whitespace

[codecov]

Codecov Report

Merging #1637 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1637      +/-   ##
==========================================
+ Coverage   91.61%   91.61%   +<.01%     
==========================================
  Files          19       20       +1     
  Lines        2099     2100       +1     
  Branches      391      391              
==========================================
+ Hits         1923     1924       +1     
  Misses        138      138              
  Partials       38       38

Impacted Files	Coverage Δ
sanic/request.py	97.58% <0%> (ø)	⬆️
sanic/app.py	92.67% <0%> (ø)	⬆️
sanic/__init__.py	100% <0%> (ø)	⬆️
sanic/blueprint_group.py	100% <0%> (ø)	⬆️
sanic/__version__.py	100% <0%> (ø)

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