Fix handling of doctest skip

[blink1073]

It turns out if you invoke an RST file directly you can't skip the doctests even with the doctest glob option, this is a workaround

[codecov-commenter]

Codecov Report

Merging #241 (b8fe154) into main (51c650e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #241   +/-   ##
=======================================
  Coverage   85.10%   85.10%           
=======================================
  Files          19       19           
  Lines        2336     2336           
  Branches      302      302           
=======================================
  Hits         1988     1988           
  Misses        251      251           
  Partials       97       97           

Impacted Files	Coverage Δ
jupyter_releaser/lib.py	83.98% <100.00%> (ø)

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