asyncio: Fix pyflakes errors

- Add a missing import
- Remove an unused import
- Remove unused variables

Doc/library/asyncio-eventloop.rst

@@ -588,10 +588,14 @@ Debug mode
 
    Get the debug mode (:class:`bool`) of the event loop, ``False`` by default.
 
+   .. versionadded:: 3.4.2
+
 .. method:: BaseEventLoop.set_debug(enabled: bool)
 
    Set the debug mode of the event loop.
 
+   .. versionadded:: 3.4.2
+
 .. seealso::
 
    The :ref:`Develop with asyncio <asyncio-dev>` section.

Lib/test/test_asyncio/test_selector_events.py

@@ -108,10 +108,7 @@ class BaseSelectorEventLoopTests(test_utils.TestCase):
         self.assertRaises(RuntimeError, self.loop.add_writer, fd, callback)
 
     def test_close_no_selector(self):
-        ssock = self.loop._ssock
-        csock = self.loop._csock
-        remove_reader = self.loop.remove_reader = mock.Mock()
-
+        self.loop.remove_reader = mock.Mock()
         self.loop._selector.close()
         self.loop._selector = None
         self.loop.close()

Lib/test/test_asyncio/test_tasks.py

 """Tests for tasks.py."""
 
-import gc
 import os.path
 import sys
 import types

Lib/test/test_asyncio/test_windows_events.py

@@ -9,6 +9,7 @@ import _winapi
 
 import asyncio
 from asyncio import _overlapped
+from asyncio import test_utils
 from asyncio import windows_events