Actually fixing #437

[henrydingliu]

undoing the previous commit on tails.ipynb. then fixing the actual issue of _config.yml not having dollarmath enabled.

tested on a new RTD instance based on my repo

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.44%. Comparing base (3ffd5c6) to head (e21bf78).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #631   +/-   ##
=======================================
  Coverage   84.44%   84.44%           
=======================================
  Files          84       84           
  Lines        4823     4823           
  Branches      610      610           
=======================================
  Hits         4073     4073           
  Misses        538      538           
  Partials      212      212           

Flag	Coverage Δ
unittests	84.44% <ø> (ø)

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