Use threadpool for reading from file in sendfile fallback mode (#14076)

diff --git a/Lib/asyncio/base_events.py b/Lib/asyncio/base_events.py
index 90de858..14b80bd 100644
--- a/Lib/asyncio/base_events.py
+++ b/Lib/asyncio/base_events.py
@@ -1141,7 +1141,7 @@
                     if blocksize <= 0:
                         return total_sent
                 view = memoryview(buf)[:blocksize]
-                read = file.readinto(view)
+                read = await self.run_in_executor(None, file.readinto, view)
                 if not read:
                     return total_sent  # EOF
                 await proto.drain()