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/locks.py

@@ -3,12 +3,13 @@
 __all__ = ('Lock', 'Event', 'Condition', 'Semaphore', 'BoundedSemaphore')
 
 import collections
+import types
 import warnings
 
 from . import events
 from . import futures
 from . import exceptions
-from .coroutines import coroutine
+from .import coroutines
 
 
 class _ContextManager:
@@ -55,7 +56,7 @@ class _ContextManagerMixin:
         # always raises; that's how the with-statement works.
         pass
 
-    @coroutine
+    @types.coroutine
     def __iter__(self):
         # This is not a coroutine.  It is meant to enable the idiom:
         #
@@ -78,6 +79,9 @@ class _ContextManagerMixin:
         yield from self.acquire()
         return _ContextManager(self)
 
+    # The flag is needed for legacy asyncio.iscoroutine()
+    __iter__._is_coroutine = coroutines._is_coroutine
+
     async def __acquire_ctx(self):
         await self.acquire()
         return _ContextManager(self)