another RTD config fix

[newville]

OK, this time for sure! See the discussion at #236 too.

One can enable RTD rendering on PRs: https://docs.readthedocs.io/en/stable/guides/pull-requests.html. That needs Admin access to the RTD config, which only @lebigot has (maybe we can fix this?).

In the meantime, I enabled RTD rendering on my own fork of uncertainties, and there I can set "build on PRs".
With this PR, the docs do build, with results at https://test-uncertainties.readthedocs.io/en/latest/

I'll suggest that as a better approach to avoiding the sort of flailing around I was doing over the past couple of days:

 when working on doc changes, enable test rendering of the RTD docs from a PR from 
 a personal fork and demonstrate that the build is successful as part of the  PR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 48.29%. Comparing base (5e54598) to head (ecdf337).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #238   +/-   ##
=======================================
  Coverage   48.29%   48.29%           
=======================================
  Files          12       12           
  Lines        1903     1903           
=======================================
  Hits          919      919           
  Misses        984      984           

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