Lazy import asyncio.sleep as it's expensive (#450)

On my system, importing tenacity (_without tornado_) takes 35ms, and
asyncio is singlehandedly responsible for 15ms. Some users do not ever
use AsyncRetrying (or asyncio in their project generally) and it would
be a shame for them to incur a unnecessary import penalty.

Full disclaimer: I pursued this change primarily to reduce pip's startup
time where asyncio was a nontrivial portion of the import timeline.

tenacity/_asyncio.py

@@ -18,7 +18,6 @@
 import functools
 import sys
 import typing as t
-from asyncio import sleep
 
 from tenacity import AttemptManager
 from tenacity import BaseRetrying
@@ -31,11 +30,20 @@
 WrappedFn = t.TypeVar("WrappedFn", bound=t.Callable[..., t.Awaitable[t.Any]])
 
 
+def asyncio_sleep(duration: float) -> t.Awaitable[None]:
+    # Lazy import asyncio as it's expensive (responsible for 25-50% of total import overhead).
+    import asyncio
+
+    return asyncio.sleep(duration)
+
+
 class AsyncRetrying(BaseRetrying):
     sleep: t.Callable[[float], t.Awaitable[t.Any]]
 
     def __init__(
-        self, sleep: t.Callable[[float], t.Awaitable[t.Any]] = sleep, **kwargs: t.Any
+        self,
+        sleep: t.Callable[[float], t.Awaitable[t.Any]] = asyncio_sleep,
+        **kwargs: t.Any,
     ) -> None:
         super().__init__(**kwargs)
         self.sleep = sleep