Clarify and fix assertions that mocks have not been awaited (GH-18196)

- The gc.collect is needed for other implementations, such as pypy
- Using context managers over multiple lines will only catch the warning from the first line in the context!
- remove a skip for a test that no longer fails on pypy

Lib/unittest/test/testmock/testmagicmethods.py

@@ -1,7 +1,6 @@
 import math
 import unittest
 import os
-import sys
 from asyncio import iscoroutinefunction
 from unittest.mock import AsyncMock, Mock, MagicMock, _magics
 
@@ -429,7 +428,6 @@ class TestMockingMagicMethods(unittest.TestCase):
             self.assertEqual(dir(mock), ['foo'])
 
 
-    @unittest.skipIf('PyPy' in sys.version, "This fails differently on pypy")
     def test_bound_methods(self):
         m = Mock()