Updated .readthedocs.yml to be python 3.8

[williamjameshandley]

Description

This increments to 3.8 in the .readthedocs.yml file, which should hopefully fix the read the docs builds

Checklist:

• I have performed a self-review of my own code
• My code is PEP8 compliant (flake8 anesthetic tests)
• My code contains compliant docstrings (pydocstyle --convention=numpy anesthetic)
• New and existing unit tests pass locally with my changes (python -m pytest)
• I have added tests that prove my fix is effective or that my feature works

[codecov]

Codecov Report

Merging #247 (41c11d8) into master (57a1afd) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #247   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           26        26           
  Lines         2271      2271           
=========================================
  Hits          2271      2271           

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