Fix warnings for "@asyncio.coroutine" usage in library

[prkumar]

This PR removes @asyncio.coroutine usage in Uplink.

#203

[codecov]

Codecov Report

Merging #248 (fc0ce44) into master (5158457) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #248   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           44        44           
  Lines         2475      2464   -11     
  Branches       358       358           
=========================================
- Hits          2475      2464   -11     

Impacted Files	Coverage Δ
uplink/clients/aiohttp_.py	100.00% <100.00%> (ø)
uplink/clients/io/asyncio_strategy.py	100.00% <100.00%> (ø)

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[prkumar]

Still need to remove usage of "asyncio.coroutine" in unit test. This will require us to separate tests for the AiohttpClient from other clients. We will also need to prevent tests for AiohttpClient and AsyncioStrategy from running when python version is 2.7

[prkumar]

Pytest documentation suggests that we can configure pytest to ignore certain test files/directories based on python version: https://docs.pytest.org/en/6.2.x/example/pythoncollection.html#customizing-test-collection