docs: Add asyncio source code links (GH-16640)

(cherry picked from commit f900064ac4b35226caad7502abc8a7e64f1c0e9d)

Co-authored-by: Kyle Stanley <aeros167@gmail.com>
diff --git a/Doc/library/asyncio-eventloop.rst b/Doc/library/asyncio-eventloop.rst
index f763fd5..1f4f0b6 100644
--- a/Doc/library/asyncio-eventloop.rst
+++ b/Doc/library/asyncio-eventloop.rst
@@ -5,6 +5,10 @@
 Event Loop
 ==========
 
+**Source code:** :source:`Lib/asyncio/events.py`,
+:source:`Lib/asyncio/base_events.py`
+
+------------------------------------
 
 .. rubric:: Preface