Add RTD config file to silence emailed deprecation warnings

[blag]

RTD has sent out multiple deprecation notices regarding config files and options in the config file.

Adding this config file should silence them.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.69%. Comparing base (4b0701b) to head (295e909).
Report is 25 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #333   +/-   ##
=======================================
  Coverage   92.69%   92.69%           
=======================================
  Files          15       15           
  Lines        1424     1424           
=======================================
  Hits         1320     1320           
  Misses        104      104           

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