[Doc] Fix missing Python class member documentation

[speth]

Sphinx 1.8 deprecated the 'autodoc_default_flags' option. It appears to have stopped working without warning in more recent versions, which means that the API docs don't include any of the member functions / member variables for any of the Python classes.

Changes proposed in this pull request

• Use autodoc_default_options instead
• Set the minimum Sphinx version to 2.0, since that's what this option requires

Checklist

• There is a clear use-case for this code change
• The commit message has a short title & references relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• The pull request is ready for review