NotGiven

For parameters with a meaningful None value, we need to distinguish between the user explicitly passing None, and the user not passing the parameter at all.

User code shouldn't need to use not_given directly.

For example:

def create(timeout: Timeout | None | NotGiven = not_given): ...

create(timeout=1)  # 1s timeout
create(timeout=None)  # No timeout
create()  # Default timeout behavior