Allow custom directory in place of '_async'

[sethmlarson]

This allows the Hip project to use ahip -> hip.

[codecov]

Codecov Report

Merging #54 into master will increase coverage by 0.19%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #54      +/-   ##
==========================================
+ Coverage   96.77%   96.96%   +0.19%     
==========================================
  Files           4        4              
  Lines         186      198      +12     
  Branches       46       46              
==========================================
+ Hits          180      192      +12     
  Misses          2        2              
  Partials        4        4

Impacted Files	Coverage Δ
src/unasync/__init__.py	96.93% <100%> (+0.19%)	⬆️
unasync/__init__.py	96.93% <0%> (+0.19%)	⬆️

[pquentin]

Thanks for tackling this! The code looks good and appears to work well

Since we're about to modify the public API, I think it would make sense to see how we're going to handle tests first, because that could affect the API too. I've opened python-trio/hip#198 for that purpose.

[pquentin]

You know what? We can always adjust the API later. Thanks for this, Seth!