Attempt to fix deprecation for RTD.

[vprusso]

Fixes deprecation from https://blog.readthedocs.com/use-build-os-config/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ac99065) 96.5% compared to head (ea145f5) 96.5%.

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #253   +/-   ##
======================================
  Coverage    96.5%   96.5%           
======================================
  Files         145     145           
  Lines        2950    2950           
  Branches      727     727           
======================================
  Hits         2848    2848           
  Misses         59      59           
  Partials       43      43           

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