bpo-33649: Fix markup; add another note that asyncio.run is 3.7+ (GH-9389)

(cherry picked from commit b042cf10c6)

Co-authored-by: Yury Selivanov <yury@magic.io>

Doc/library/asyncio-task.rst

@@ -20,7 +20,8 @@ Coroutines
 
 Coroutines declared with async/await syntax is the preferred way of
 writing asyncio applications.  For example, the following snippet
-of code prints "hello", waits 1 second, and then prints "world"::
+of code (requires Python 3.7+) prints "hello", waits 1 second,
+and then prints "world"::
 
     >>> import asyncio
 

Doc/library/asyncio.rst

@@ -8,7 +8,7 @@
 
 .. sidebar:: Hello World!
 
-   .. code-block:: python
+   ::
 
        import asyncio