Improve handling of dev versions

[blink1073]

Working toward #272. Allows a simplified handling of dev mode, using the following config:

[tool.jupyter-releaser.options]
post-version-spec = "dev"

[codecov-commenter]

Codecov Report

Merging #287 (aa77d23) into main (d700656) will decrease coverage by 0.19%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##             main     #287      +/-   ##
==========================================
- Coverage   84.69%   84.50%   -0.20%     
==========================================
  Files          19       19              
  Lines        2424     2413      -11     
  Branches      309      306       -3     
==========================================
- Hits         2053     2039      -14     
- Misses        269      271       +2     
- Partials      102      103       +1     

Impacted Files	Coverage Δ
jupyter_releaser/changelog.py	84.74% <66.66%> (-0.97%)	⬇️
jupyter_releaser/util.py	70.49% <94.44%> (+0.70%)	⬆️
jupyter_releaser/cli.py	90.87% <100.00%> (-0.62%)	⬇️
jupyter_releaser/lib.py	84.10% <100.00%> (ø)
jupyter_releaser/tests/test_functions.py	98.93% <100.00%> (+0.06%)	⬆️
jupyter_releaser/tests/conftest.py	98.40% <0.00%> (-0.16%)	⬇️
jupyter_releaser/tests/test_cli.py	99.04% <0.00%> (-0.01%)	⬇️
jupyter_releaser/tests/util.py	99.28% <0.00%> (-0.01%)	⬇️

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