Add support for Python 3.8 specifics

• Allow alternative results when testing spec.md examples (depending under which Python version Paroxython runs).
• Extend no_operation to Ellipsis syntax.
• Add assignment_expression feature.
• Correct the span of decorated functions and classes.
• Make flatten_ast.py to work with both typed-ast and ast library.
• Add a helper to compare the results of their respective parser.
• Upgrade the development environment to Python 3.8.