Improve test speed

[blink1073]

Follow advice from https://github.com/zupo/awesome-pytest-speedup

Locally the full test suite went from 200s -> 56s.

[codecov-commenter]

Codecov Report

Base: 81.16% // Head: 81.27% // Increases project coverage by +0.11% 🎉

Coverage data is based on head (7a53af6) compared to base (7977e0e).
Patch coverage: 76.47% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #445      +/-   ##
==========================================
+ Coverage   81.16%   81.27%   +0.11%     
==========================================
  Files          20       20              
  Lines        2824     2831       +7     
  Branches      365      366       +1     
==========================================
+ Hits         2292     2301       +9     
+ Misses        399      397       -2     
  Partials      133      133              

Impacted Files	Coverage Δ
jupyter_releaser/python.py	72.61% <ø> (ø)
jupyter_releaser/tests/test_cli.py	98.87% <ø> (ø)
jupyter_releaser/mock_github.py	63.37% <14.28%> (-0.75%)	⬇️
jupyter_releaser/tests/conftest.py	97.48% <60.00%> (-1.26%)	⬇️
jupyter_releaser/lib.py	79.81% <100.00%> (+1.80%)	⬆️
jupyter_releaser/util.py	70.02% <100.00%> (+0.30%)	⬆️

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