fix wrong signing key usage

[nichtsfrei]

What:

pontos was using signing-key instead of git-signing-key within prepare.

Why:

How:

Checklist:

• Tests
• CHANGELOG Entry
• Documentation

[codecov]

Codecov Report

Merging #42 into master will increase coverage by 0.19%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
+ Coverage   93.11%   93.31%   +0.19%     
==========================================
  Files          26       26              
  Lines        1176     1196      +20     
==========================================
+ Hits         1095     1116      +21     
+ Misses         81       80       -1     

Impacted Files	Coverage Δ
pontos/release/release.py	96.11% <ø> (+0.55%)	⬆️
tests/release/test_release.py	98.90% <100.00%> (+0.13%)	⬆️

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