Remove use of deprecated ast classes and methods

[mschwager]

Per the Python 3.8 release notes:

ast classes Num, Str, Bytes, NameConstant and Ellipsis are considered deprecated and will be removed in future Python versions. Constant should be used instead. (Contributed by Serhiy Storchaka in bpo-32892.)

ast.NodeVisitor methods visit_Num(), visit_Str(), visit_Bytes(), visit_NameConstant() and visit_Ellipsis() are deprecated now and will not be called in future Python versions. Add the visit_Constant() method to handle all constant nodes. (Contributed by Serhiy Storchaka in bpo-36917.)

This isn't immediately necessary, but it's worth tracking. It's not clear when this change will happen, so we don't need a definite timeline. If we can wait until Python 2.7 support is dropped (#17) then this change will be a bit easier since many of these classes/methods are used in 2.7. For example, Dlint makes use of ast.Str and ast.NameConstant.

[clavedeluna]

Can be done once all versions including py3.7 are deprecated