bpo-38692: Add asyncio.PidfdChildWatcher to __all__ (GH-17161)



/cc @asvetlov @1st1 


https://bugs.python.org/issue38692



Automerge-Triggered-By: @benjaminp
diff --git a/Lib/asyncio/unix_events.py b/Lib/asyncio/unix_events.py
index d02460c..6621372 100644
--- a/Lib/asyncio/unix_events.py
+++ b/Lib/asyncio/unix_events.py
@@ -29,7 +29,7 @@
 __all__ = (
     'SelectorEventLoop',
     'AbstractChildWatcher', 'SafeChildWatcher',
-    'FastChildWatcher',
+    'FastChildWatcher', 'PidfdChildWatcher',
     'MultiLoopChildWatcher', 'ThreadedChildWatcher',
     'DefaultEventLoopPolicy',
 )