Clean up warnings triggered by python 3.8

[jenshnielsen]

This cleans up new warnings triggered by python 3.8

• Don't use is to compare against string literals
• Don't use deprecated and unused encoding kwarg for json.loads. This has been unused since 3.1
• Stop using deprecated @coroutine decorator
• Remove pre python 3.5 workarounds in coroutine tests

[codecov]

Codecov Report

Merging #1723 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1723   +/-   ##
=======================================
  Coverage   67.31%   67.31%           
=======================================
  Files         146      146           
  Lines       18072    18072           
=======================================
  Hits        12165    12165           
  Misses       5907     5907

[jenshnielsen]

There are still a number of warnings. They are as far as I can see triggered from dependencies and will hopefully be resolved upstream soon.

[WilliamHPNielsen]

Looks good! About time we drop support for pre-py3.5 syntax :)