refactor(typing): use Literal type hint for conflict attribute

[sisp]

I've made the type hint for the conflict attribute more precise by using Literal["rej", "inline"] instead of str.

@yajo Is refactor(typing) the correct change type in your opinion? It seemed the most appropriate to me, but I couldn't find a guideline for committing only typing changes.

[codecov]

Codecov Report

Merging #1200 (5d4d0fe) into master (1b351bc) will increase coverage by 0.17%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1200      +/-   ##
==========================================
+ Coverage   96.74%   96.92%   +0.17%     
==========================================
  Files          47       47              
  Lines        3935     3935              
==========================================
+ Hits         3807     3814       +7     
+ Misses        128      121       -7     

Flag	Coverage Δ
unittests	96.92% <100.00%> (+0.17%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
copier/main.py	95.64% <100.00%> (ø)

... and 2 files with indirect coverage changes

[yajo]

I relabeled the commit to the build category. Refactor commits appear in the changelog, and I don't think this one deserves such a honor. 😜