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/test/test_asyncio/test_windows_utils.py

@@ -9,9 +9,9 @@ from unittest import mock
 if sys.platform != 'win32':
     raise unittest.SkipTest('Windows only')
 
+import _overlapped
 import _winapi
 
-from asyncio import _overlapped
 from asyncio import windows_utils
 try:
     from test import support