bpo-39031: Include elif keyword when producing lineno/col-offset info for if_stmt (GH-17582)

When parsing an "elif" node, lineno and col_offset of the node now point to the "elif" keyword and not to its condition, making it consistent with the "if" node.


https://bugs.python.org/issue39031



Automerge-Triggered-By: @pablogsal