Allow opting in to treat datetimeus as datetime rather than DatetimeUS

For new applications/databases it might make sense to not have to deal with this historic artifact. DatetimeUS can trip up some downstream libraries like pydantic/fastapi.

Start by locating where the Python API maps the database type `datetimeus` to `DatetimeUS`, then inspect any related tests or type-handling entry points. Done means a documented opt-in for new applications or databases that treats `datetimeus` as `datetime` without changing the existing default behavior.

Written by the indexing model from the issue text.