FIX: exit if non-html builder

[agoose77]

No description provided.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (7bf0c3f) 90.71% compared to head (c171755) 90.59%.

Files	Patch %	Lines
sphinx_external_toc/events.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #104      +/-   ##
==========================================
- Coverage   90.71%   90.59%   -0.13%     
==========================================
  Files           7        7              
  Lines         743      744       +1     
==========================================
  Hits          674      674              
- Misses         69       70       +1     

Flag	Coverage Δ
pytests	90.59% <0.00%> (-0.13%)	⬇️

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