Add support for attrs factories taking self.

[python-desert]

No description provided.

[codecov-io]

Codecov Report

Merging #27 into master will increase coverage by 0.63%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #27      +/-   ##
==========================================
+ Coverage   95.85%   96.48%   +0.63%     
==========================================
  Files           6        6              
  Lines         362      370       +8     
  Branches       66       67       +1     
==========================================
+ Hits          347      357      +10     
+ Misses          9        8       -1     
+ Partials        6        5       -1

Impacted Files	Coverage Δ
tests/test_make.py	98.04% <100%> (+0.07%)	⬆️
src/desert/_version.py	100% <100%> (ø)	⬆️
src/desert/_make.py	93.02% <100%> (+1.55%)	⬆️

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