Type Hint Improvements

[Atlamillias]

This PR corrects a few Python type annotations and argument defaults to reflect the hinted type within function signatures. The notable changes include:

• context manager functions now return Iterator[int | str]
• the label keyword argument is now Optional[str]
• the event_type keyword argument for add_item_focus_handler() and add_item_hover_handler() now defaults to mvEventType_On instead of None

Concerning Areas:
Dear PyGui uses several deprecated symbols within the typing module. This PR adds Iterator and Optional to that list. Iterator can instead be imported from collections.abc. This is not possible with Optional, however, as the equivalent as of Python 3.10 is T | None.