bpo-32410: Make SendfileNotAvailableError exception public (#5243)

diff --git a/Lib/asyncio/events.py b/Lib/asyncio/events.py
index b06721f..d5365dc 100644
--- a/Lib/asyncio/events.py
+++ b/Lib/asyncio/events.py
@@ -3,7 +3,7 @@
 __all__ = (
     'AbstractEventLoopPolicy',
     'AbstractEventLoop', 'AbstractServer',
-    'Handle', 'TimerHandle',
+    'Handle', 'TimerHandle', 'SendfileNotAvailableError',
     'get_event_loop_policy', 'set_event_loop_policy',
     'get_event_loop', 'set_event_loop', 'new_event_loop',
     'get_child_watcher', 'set_child_watcher',
@@ -20,6 +20,14 @@
 from . import format_helpers
 
 
+class SendfileNotAvailableError(RuntimeError):
+    """Sendfile syscall is not available.
+
+    Raised if OS does not support senfile syscall for given socket or
+    file type.
+    """
+
+
 class Handle:
     """Object returned by callback registration methods."""