fix: mypy errors when using tomlkit to modify toml document

[waketzheng]

Fixes #326

[waketzheng]

Mini demo to check result:

from pathlib import Path
import tomlkit

doc = tomlkit.parse(Path('pyproject.toml').read_text())
poetry_conf = doc['tool']['poetry']
print(poetry_conf)
poetry_conf['version'] = '0'

[frostming]

But that is not right, you can't assume an abitrary item is a table.

You can't make that assumption in vanilla python either:

doc: dict[str, Any] = get_data()
print(doc['tool']['abc'])  # what type is doc['tool']?

Instead, you have to tell the typechecker the type information.