add ipywidgets dependency

[AurelienBoland]

This dependency is necessary for the progress bar

[codecov]

Codecov Report

Merging #241 (789e786) into master (46fc0e5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #241   +/-   ##
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  Coverage   91.37%   91.37%           
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  Files          12       12           
  Lines         580      580           
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  Hits          530      530           
  Misses         50       50           

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[nalepae]

Only if you use a notebook:
https://nalepae.github.io/pandarallel/troubleshooting/

We should not add this dependancies for users using pandarallel outside of notebook.
An other idea would be to add an "extra" containing ipywidget