Add Python 3.6.8 code compatibility modifications

[hderry]

• typing.Self doesn't exist until Python 3.11--> fix import line
• Module-level dict annotation -> Subscripting built-in types like dict[str, int] (PEP 585) needs Python 3.9+
• Function parameter annotation -> Function annotations are also evaluated at def time on 3.6
• Change return type using Self