Add readthedocs config file to force newer dependencies

[djhoese]

It was pointed out on slack that there were some formatting issues in the installation instructions (the last pip install -e . wasn't showing up). It looks like readthedocs defaults to using really old versions for projects created before 2020.

• Closes #xxxx (remove if there is no corresponding issue, which should only be the case for minor changes)
• Tests added (for all bug fixes or enhancements)
• Tests passed (for all non-documentation changes)
• Passes git diff origin/master **/*py | flake8 --diff (remove if you did not edit any Python files)
• Fully documented (remove if this change should not be visible to users, e.g., if it is an internal clean-up, or if this is part of a larger project that will be documented later)

[codecov]

Codecov Report

Merging #110 (bffedb4) into main (93e547a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #110   +/-   ##
=======================================
  Coverage   92.93%   92.93%           
=======================================
  Files          10       10           
  Lines        3495     3495           
=======================================
  Hits         3248     3248           
  Misses        247      247           

Flag	Coverage Δ
unittests	92.93% <ø> (ø)

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[mraspaud]

LGTM