Genshi doesn't support Python 3.8

[anonymous]

Python 3.8 now uses ast.Constant type and remove types like _ast.Str: ast.Str is a backward compatibility class.

I had to disable Genshi benchmark in the performance project: ​https://github.com/python/performance/issues/46

Example of issue: "strarg = _new(_ast.Str, node.id)" at genshi/template/eval.py:616: the code should use ast.Str, not _ast.Str.