Fixes exception due to ast parser changes.

[comfuture]

I just wrote quick fix of #29 by producing handle_Constant() method for each AstHandlers. And the existing handle_Num(), handle_Str(), .. methods were left untouched to obtain backward compatibility.

See https://docs.python.org/3/library/ast.html#ast.NodeVisitor.generic_visit

since version 3.8: 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.

[cjmaynar]

Would love to see this get merged in and released upstream to allow upgrades to python 3.8

[cjmaynar]

@alonho is there anyway you could merge this and release a new version on pypy to allow for python 3.8 compatability?