Added furo as extras_require in setup.py

[MatteoRobbiati]

In order to compile sphinx with the new theme, furo must be installed. I modified the setup.py file by adding it in the extras_require where we currently have sphinx_rtd_theme.

[codecov]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (dc44044) compared to base (9e535e7).
Patch has no changes to coverable lines.

Additional details and impacted files

@@                 Coverage Diff                 @@
##           Furo_Sphinx_Theme      #700   +/-   ##
===================================================
  Coverage             100.00%   100.00%           
===================================================
  Files                     84        84           
  Lines                  11015     11015           
===================================================
  Hits                   11015     11015           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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