Use mock github when in dry run mode

[blink1073]

Part of #272

[codecov-commenter]

Codecov Report

Merging #355 (01c0d9d) into main (7e04bc4) will decrease coverage by 0.52%.
The diff coverage is 51.72%.

@@            Coverage Diff             @@
##             main     #355      +/-   ##
==========================================
- Coverage   79.01%   78.48%   -0.53%     
==========================================
  Files          21       21              
  Lines        2669     2715      +46     
  Branches      347      353       +6     
==========================================
+ Hits         2109     2131      +22     
- Misses        440      467      +27     
+ Partials      120      117       -3     

Impacted Files	Coverage Δ
jupyter_releaser/actions/common.py	0.00% <0.00%> (ø)
jupyter_releaser/mock_github.py	0.00% <0.00%> (ø)
jupyter_releaser/lib.py	81.29% <90.90%> (+1.00%)	⬆️
jupyter_releaser/changelog.py	84.17% <100.00%> (-0.12%)	⬇️
jupyter_releaser/cli.py	91.51% <100.00%> (+0.06%)	⬆️
jupyter_releaser/tests/conftest.py	98.56% <100.00%> (-0.02%)	⬇️
jupyter_releaser/util.py	72.34% <100.00%> (+2.82%)	⬆️

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