Migrate code to python3.9

[kindermax]

Since we drop support for python3.8 and now minimal python version is python3.9, we can migrate some library code to new python3.9 types and functions, which includes:

• builtin container types now can be used in type annotations e.g. List[] -> list[]
• from collections.abc import Iterable, Mapping, Sequence
• maybe some types migrated from typing_extensions into typing module

Also review python3.9 official changelog, maybe we can use some new features https://docs.python.org/3.9/whatsnew/3.9.html