Set new required build fields in the ReadTheDocs config file

[micahellison]

ReadTheDocs is warning us that we need to add build.os to our configuration file before October 16. This PR makes the change by selecting the latest available Ubuntu version.

Edit: Turns out they need build.tools.python as well, so I'm setting that here too. I think setting to "3" for the latest version of Python 3 is acceptable for documentation builds, rather than pinning to a specific version, but I'm open to changing it.

Checklist

• I have read the contributing doc.
• I have included a link to the relevant issue number.
• I have checked to ensure there aren't other open pull requests
  for the same issue.
• I have written new tests for these changes, as needed.