MAINT: update entry-points usage

[agoose77]

This PR updates our entry-points usage, noticed by @bollwyvl in https://github.com/bollwyvl/jupyter-book-feedstock/commit/f5729051ba84cadf880a1b384a1fd8658b8a1824

[codecov[bot]]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (30afdb7) 91.30% compared to head (e001397) 91.43%. Report is 6 commits behind head on master.

Files	Patch %	Lines
jupyter_book/cli/pluggable.py	40.00%	3 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2121      +/-   ##
==========================================
+ Coverage   91.30%   91.43%   +0.13%     
==========================================
  Files           7        7              
  Lines         690      689       -1     
==========================================
  Hits          630      630              
+ Misses         60       59       -1     

Flag	Coverage Δ
pytests	91.43% <40.00%> (+0.13%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.