Fix handling of "since" parameter

[fcollonval]

Fixes capture of RH_SINCE that was disrupted by #106

[codecov-commenter]

Codecov Report

Merging #110 (f9f69b1) into master (b1ab320) will decrease coverage by 0.16%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #110      +/-   ##
==========================================
- Coverage   86.34%   86.18%   -0.17%     
==========================================
  Files          17       17              
  Lines        2051     2055       +4     
  Branches      242      244       +2     
==========================================
  Hits         1771     1771              
- Misses        191      195       +4     
  Partials       89       89              

Impacted Files	Coverage Δ
jupyter_releaser/actions/draft_release.py	0.00% <0.00%> (ø)

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[blink1073]

Thanks!