Update tracing library to be python 3.9 compatible.

`asyncio.Task.current_task()` was deprecated in python 3.7 and removed in 3.9.
This was replaced by `asyncio.current_task()`.
https://docs.python.org/3.8/library/asyncio-task.html#asyncio.Task.current_task

PiperOrigin-RevId: 374223589

tensorflow_federated/python/common_libs/tracing.py

@@ -286,10 +286,16 @@ _non_async_span_yields = ThreadLocalSpanYields()
 
 
 def _current_task() -> Optional[asyncio.Task]:
+  """Get the current running task, or `None` if no task is running."""
+  # Note: `current_task` returns `None` if there is no current task, but it
+  # throws if no currently running async loop.
   try:
-    # Note: `current_task` returns `None` if there is no current task,
-    # but it throws if no currently running async loop.
-    return asyncio.Task.current_task()
+    # asyncio.Task.current_task was moved to asyncio.current_task() starting
+    # with 3.9, but previoulsy was asyncio.Task.current() until Python 3.6.
+    if sys.version_info[1] >= 7:
+      return asyncio.current_task()
+    else:
+      return asyncio.Task.current_task()
   except RuntimeError:
     return None