Convert dependency exclusion to marker format

[techalchemy]

• Fixes Packaging dependency exclusions should be handled using markers #14

[codecov]

Codecov Report

Merging #15 into master will increase coverage by 1.68%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #15      +/-   ##
==========================================
+ Coverage   96.55%   98.23%   +1.68%     
==========================================
  Files           5        5              
  Lines         174      170       -4     
  Branches       27       25       -2     
==========================================
- Hits          168      167       -1     
+ Misses          2        1       -1     
+ Partials        4        2       -2

Impacted Files	Coverage Δ
src/backports/os.py	96% <ø> (+2.32%)	⬆️
tests/test_helpers.py	100% <0%> (+4.16%)	⬆️

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