bpo-40275: Avoid importing asyncio in test.support (GH-19600)

* Import asyncio lazily in unittest (only when IsolatedAsyncioTestCase is used).
* Import asyncio.events lazily in test.support.

Lib/test/support/__init__.py

@@ -3,7 +3,6 @@
 if __name__ != 'test.support':
     raise ImportError('support must be imported from the test package')
 
-import asyncio.events
 import collections.abc
 import contextlib
 import errno
@@ -3260,6 +3259,7 @@ SMALLEST = _SMALLEST()
 
 def maybe_get_event_loop_policy():
     """Return the global event loop policy if one is set, else return None."""
+    import asyncio.events
     return asyncio.events._event_loop_policy
 
 # Helpers for testing hashing.