Adding OS specification to readthedocs build

[eytanadler]

Purpose

An OS for the docs build must be specified by October 16 or everything will explode.

Type of change

What types of change is it?
Select the appropriate type(s) that describe this PR

• Documentation update

Testing

Explain the steps needed to test the new code to verify that it does indeed address the issue and produce the expected behavior.

Checklist

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• I have run unit and regression tests which pass locally with my changes
• I have added new tests that prove my fix is effective or that my feature works
• I have added necessary documentation

[codecov]

Codecov Report

Merging #55 (8dd937b) into main (34f5f94) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #55   +/-   ##
=======================================
  Coverage   79.99%   79.99%           
=======================================
  Files          85       85           
  Lines        9297     9297           
=======================================
  Hits         7437     7437           
  Misses       1860     1860           

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