Support new source location fields in Python 3.8+

[Python 3.8 added](https://docs.python.org/3/whatsnew/3.8.html#ast) the `end_lineno` and `end_col_offset` fields to AST nodes that have `lineno` and `col_offset`.

Specs in the [official documentation](https://docs.python.org/3/library/ast.html#ast.AST.end_lineno).

We should probably worry about these anywhere we handle source location info.
