Greatly improved typehints

[trag1c]

1. Updated typehints to use pipe union syntax instead of typing.Optional[T] or typing.Union[X, Y]
2. Removed redundant typehints where the type is clear from context, such as x: str = "hi"
3. Converted generator typehints from Generator[T, None, None] to Iterator[T]
4. Added missing typehints
5. Improved existing typehints (mostly subscripting types)
6. Made typehints backwards compatible with 3.8+
7. Also fixed one syntax error that had a context manager using 3.9+ syntax