Fix pydantic - mypy 1.1 incompatibility

Mypy 1.1.1 throws errors with pydantic models that specify default values with
positional arguments. The next pydantic version will probably fix it by updating
the mypy-pydantic plugin, but converting the default value to a keyword argument
is no big deal either. So let's fix it today.

For more info pydantic/pydantic#5077