ci: add depency for librosa

[deepcharles]

A notebook example needs librosa and librosa needs a library that cannot be installed with pip (see here).

We add the dependency to the gh actions that generate the docs.

[codecov]

Codecov Report

Merging #121 (7bdfe08) into master (68074b7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #121   +/-   ##
=======================================
  Coverage   95.12%   95.12%           
=======================================
  Files          41       41           
  Lines         965      965           
=======================================
  Hits          918      918           
  Misses         47       47           

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