Update RTD configuration

[kandersolar]

• Closes #xxx
• Added tests to cover all new or modified code.
• Clearly documented all new API functions with PEP257 and numpydoc compliant docstrings.
• Added new API functions to docs/api.rst.
• Non-API functions clearly documented with docstrings or comments as necessary.
• Adds description and name entries in the appropriate "what's new" file
  in docs/whatsnew
  for all changes. Includes link to the GitHub Issue with :issue:`num`
  or this Pull Request with :pull:`num`. Includes contributor name
  and/or GitHub username (link with :ghuser:`user`).
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels and Milestone are assigned to the Pull Request and linked Issue.

ReadTheDocs is requiring a change in the configuration file (see the warning on this build):

Your builds will stop working soon!
"build.image" config key is deprecated and it will be removed soon. Read our blog post to know how to migrate to new key "build.os" and ensure your project continues building successfully.

This PR makes the necessary configuration updates.