Use obj.model_dump instead of obj.dict for Pydantic V2

[waketzheng]

Fixes complaint for blacksheep unittest:

examples/blacksheep/_tests.py::test_get_uses_list
  /Users/mac10.12/Library/Caches/pypoetry/virtualenvs/tortoise-orm-iT379CwK-py3.11/lib/python3.11/site-packages/pydantic/main.py:1114: PydanticDeprecatedSince20: The `dict` method is deprecated; use `model_dump` instead. Deprecated in Pydantic V2.0 to be removed in V3.0. See Pydantic V2 Migration Guide at https://errors.pydantic.dev/2.9/migration/
    warnings.warn('The `dict` method is deprecated; use `model_dump` instead.', category=PydanticDeprecatedSince20)

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html