py domain: Use AST parser to convert signature to doctree

[tk0miya]

Feature or Bugfix

• Feature

Purpose

• As a preparation of py domain: Allow to make a style for arguments of functions and methods #6990, Use AST parser to convert signature to doctree
• It allows to decorate each tokens in latter step.
• Originally, this is a part of py domain: Allow to make a style for arguments of functions and methods #6990.