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_events.py

@@ -2141,7 +2141,7 @@ if sys.platform == 'win32':
         def test_remove_fds_after_closing(self):
             raise unittest.SkipTest("IocpEventLoop does not have add_reader()")
 else:
-    from asyncio import selectors
+    import selectors
 
     class UnixEventLoopTestsMixin(EventLoopTestsMixin):
         def setUp(self):