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.

Doc/whatsnew/3.7.rst

@@ -677,6 +677,11 @@ Changes in Python behavior
 Changes in the Python API
 -------------------------
 
+* :mod:`asyncio`: The module doesn't export :mod:`selectors` and
+  :mod:`_overlapped` modules as ``asyncio.selectors`` and
+  ``asyncio._overlapped``. Replace ``from asyncio import selectors`` with
+  ``import selectors`` for example.
+
 * :meth:`pkgutil.walk_packages` now raises ValueError if *path* is a string.
   Previously an empty list was returned. (Contributed by Sanyam Khurana in
   :issue:`24744`.)