Parse tree to AST code for Python 3.6-3.8.

[jriehl]

The last coding push I made got stuck trying to make the AST-ifier code in Mython work exactly like Python for 3.6, 3.7, and 3.8. Looking for help with getting all test cases to pass before tackling Python 3.9 and later (which throws pgen right out the window).

Current code is in the better_astify branch.