Add type overloads for async `sleep` function

[Zac Hatfield-Dodds]

import trio  # or asyncio, etc.
import tenacity

@tenacity.retry(
    sleep=trio.sleep,  # typecheckers will error here!
)
async def foo():
    pass

The type annotation here

tenacity/tenacity/__init__.py

Lines 500 to 502 in 4333249

 @t.overload 
 def retry( 
     sleep: t.Callable[[t.Union[int, float]], None] = sleep, 

should be t.Callable[[t.Union[int, float]], t.Union[None, t.Awaitable[None]], since AsyncRetrying allows async sleep functions.

Going further, we could define an additional AsyncWrappedFn typevar and propagate that though to ensure that the sleep callback is always awaitable when retrying for a coroutine function. This is a pretty important performance footgun to avoid!