bpo-32154: Remove asyncio.selectors (#4605)

* Remove asyncio.selectors and asyncio._overlapped symbols from the
  namespace of the asyncio module
* Replace "from asyncio import selectors" with "import selectors"
* Replace "from asyncio import _overlapped" with "import _overlapped"

asyncio.selectors was added to support Python 3.3, which doesn't have
selectors in its standard library, and Python 3.4 in the same code
base. Same rationale for asyncio._overlapped. Python 3.3 reached its
end of life, and asyncio is no more maintained as a third party
module on PyPI.

Lib/asyncio/unix_events.py

@@ -2,6 +2,7 @@
 
 import errno
 import os
+import selectors
 import signal
 import socket
 import stat
@@ -18,7 +19,6 @@ from . import coroutines
 from . import events
 from . import futures
 from . import selector_events
-from . import selectors
 from . import transports
 from .coroutines import coroutine
 from .log import logger