Do not wrap async function with threaded_callback

[prkumar]

Async callbacks are being wrapped by ThreadedCoroutine when they shouldn't be. For some context, the purpose of ThreadedCoroutine is to allow sync callbacks with aiohttp client. When the callback is an async function, this condition should skip this behavior.

Fixes #256

[codecov]

Codecov Report

Merging #258 (c86044a) into master (d9aa5b2) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #258   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           44        44           
  Lines         2464      2471    +7     
  Branches       358       358           
=========================================
+ Hits          2464      2471    +7     

Impacted Files	Coverage Δ
uplink/builder.py	100.00% <100.00%> (ø)
uplink/clients/aiohttp_.py	100.00% <100.00%> (ø)
uplink/compat.py	100.00% <100.00%> (ø)
uplink/hooks.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d9aa5b2...c86044a. Read the comment docs.