add __slots__ and improved performance using lower level asyncio.Future instead of asyncio.Event and typehint the library

[Vizonex]

I saw this library and knew it had potential with some of my upcoming python libraries I have planned. Kind of surprised it's not getting the attention and love that it really deserves so I added a few performance improvements for some internal refactoring.

• I got rid of asyncio.Event in trade for asyncio.Future and I added in _loop for creating these objects as a speedboost.
• added __slots__ to class attributes to impove speed and strictness of the class since really I don't people should subclass but feel free to prove me wrong with that one.
• fixed pytest module since the last test triggers a warning popup that I got rid of for you.
• Last but not least as the cherry on top, I made sure that the library was typehinted and added py.typed so that contributors to typeshead will ignore this burden. (and I tested it with mypy to ensure it's safety elsewhere).
• Made sure 3.9 would still be supported up until it hits End Of Life