Drop python3.8 support

[waketzheng]

1. Drop py38 support;
2. Run make up to upgrade dependencies;
3. Move config of mypy from setup.cfg to pyproject.toml;
4. Bump up actions version in pypi.yml;
5. Add section about config set in pyproject.toml to readme;
6. Use py38 syntax to refactor tortoise_cli/cli.py;
7. Import version value from pyproject.toml instead of hard code in tortoise_cli/init.py

[waketzheng]

@long2ice Could you merge it?