Only run check-manifest if using setuptools

[blink1073]

Fixes #280

[codecov-commenter]

Codecov Report

Merging #302 (925962f) into main (703fbbe) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #302   +/-   ##
=======================================
  Coverage   83.86%   83.87%           
=======================================
  Files          19       19           
  Lines        2529     2530    +1     
  Branches      336      336           
=======================================
+ Hits         2121     2122    +1     
  Misses        293      293           
  Partials      115      115           

Impacted Files	Coverage Δ
jupyter_releaser/cli.py	91.45% <100.00%> (ø)
jupyter_releaser/util.py	69.59% <100.00%> (+0.11%)	⬆️

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