Fix issue with setup requirements not being installed

[seppeljordan]

This should fix #276

[codecov-io]

Codecov Report

Merging #289 into master will increase coverage by 0.03%.
The diff coverage is 40%.

@@            Coverage Diff             @@
##           master     #289      +/-   ##
==========================================
+ Coverage   67.81%   67.85%   +0.03%     
==========================================
  Files          22       22              
  Lines        1544     1549       +5     
==========================================
+ Hits         1047     1051       +4     
- Misses        497      498       +1

Impacted Files	Coverage Δ
src/pypi2nix/stage1.py	90.69% <40%> (-0.67%)	⬇️

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