Rename .readthedocs.yml to .readthedocs.yaml

[graingert]

Apparently both are supported, and yaml is the preferred extension

Contributor Checklist:

• [ ] I have updated the release notes at docs/source/NEWS.rst
• [x] I have updated the automated tests.
• [ ] All tests pass on your local dev environment. See CONTRIBUTING.rst.

[codecov[bot]]

Codecov Report

Merging #215 (3673945) into master (2e1d923) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #215   +/-   ##
=======================================
  Coverage   83.30%   83.30%           
=======================================
  Files          87       87           
  Lines       11569    11569           
  Branches     1184     1184           
=======================================
  Hits         9637     9637           
  Misses       1815     1815           
  Partials      117      117           

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[psi29a]

Looks like prettier fails?

[graingert]

Looks like prettier fails?

fixed