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/selector_events.py

@@ -9,6 +9,7 @@ __all__ = ['BaseSelectorEventLoop']
 import collections
 import errno
 import functools
+import selectors
 import socket
 import warnings
 import weakref
@@ -21,7 +22,6 @@ from . import base_events
 from . import constants
 from . import events
 from . import futures
-from . import selectors
 from . import transports
 from . import sslproto
 from .coroutines import coroutine