Add aiofiles.os.link function

[jpy-git]

Fixes #117.

The os module in the stdlib provides a function called os.link that creates a hardlink to the src file. This PR adds the async version of that.

Docs: https://docs.python.org/3/library/os.html#os.link

I've added the relevant unit test in test_os.py

[codecov-commenter]

Codecov Report

Merging #121 (70d876a) into master (d010ff4) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #121      +/-   ##
==========================================
+ Coverage   89.59%   89.63%   +0.03%     
==========================================
  Files          10       10              
  Lines         298      299       +1     
==========================================
+ Hits          267      268       +1     
  Misses         31       31              

Impacted Files	Coverage Δ
src/aiofiles/os.py	100.00% <100.00%> (ø)

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[Tinche]

Thank you!