bpo-36921: Deprecate @coroutine for sake of async def (GH-13346)

The second attempt. Now deprecate `@coroutine` only, keep `yield from fut` as is.


https://bugs.python.org/issue36921

Lib/asyncio/coroutines.py

@@ -7,6 +7,7 @@ import os
 import sys
 import traceback
 import types
+import warnings
 
 from . import base_futures
 from . import constants
@@ -107,6 +108,9 @@ def coroutine(func):
     If the coroutine is not yielded from before it is destroyed,
     an error message is logged.
     """
+    warnings.warn('"@coroutine" decorator is deprecated since Python 3.8, use "async def" instead',
+                  DeprecationWarning,
+                  stacklevel=2)
     if inspect.iscoroutinefunction(func):
         # In Python 3.5 that's all we need to do for coroutines
         # defined with "async def".