Issue #21907: Make the buildbot clean script always return 0.

The clean script is a "best effort" thing anyway, and this will hopefully
revive the XP buildbot.
Issue #21680: Document asyncio event loops
(Merge 3.4) Issue #21680: Document asyncio event loops
asyncion, Tulip issue 181: BaseEventLoop.create_datagram_endpoint() now waits
until protocol.connection_made() has been called. Document also why transport
constructors use a waiter.
(Merge 3.4) asyncion, Tulip issue 181: BaseEventLoop.create_datagram_endpoint()
now waits until protocol.connection_made() has been called. Document also why
transport constructors use a waiter.
Issue #21938: simplify gen_iternext()
Fix bootstrapping asdl -- it didn't work with Python 2.7.
Issue #6916: Use assertWarns in test_asynchat.