Fix to readthedocs builds

[freemansw1]

Resolves an issue with readthedocs builds discovered with #335.

We, incidentally, talked about this today. See: https://blog.readthedocs.com/drop-support-system-packages/ . While we had to deal with some of the consequences of this earlier, we now have the new issue of system_packages: true in our readthedocs config, breaking readthedocs builds. This PR should alleviate that issue.

In my mind, this is now a blocking PR, and 1.5.1 shouldn't be released until this is merged or the issue is resolved in some other way.

• Have you followed our guidelines in CONTRIBUTING.md?
• Have you self-reviewed your code and corrected any misspellings?
• Have you written documentation that is easy to understand?
• Have you written descriptive commit messages?
• Have you added NumPy docstrings for newly added functions?
• Have you formatted your code using black?
• If you have introduced a new functionality, have you added adequate unit tests?
• Have all tests passed in your local clone?
• If you have introduced a new functionality, have you added an example notebook?
• Have you kept your pull request small and limited so that it is easy to review?
• Have the newest changes from this branch been merged?

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (a23a220) 55.74% compared to head (0fab557) 55.74%.

Additional details and impacted files

@@               Coverage Diff               @@
##           RC_v1.5_hotfix     #336   +/-   ##
===============================================
  Coverage           55.74%   55.74%           
===============================================
  Files                  15       15           
  Lines                3310     3310           
===============================================
  Hits                 1845     1845           
  Misses               1465     1465           

Flag	Coverage Δ
unittests	55.74% <ø> (ø)

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[w-k-jones]

Looks good, readthedocs is also generating properly including the API references. I agree this should be part of v1.5.1 release

[w-k-jones]

Given that this is definitely working, I'm happy for this to be merged with a single review if you are @freemansw1