`Task.current_task()` and `Task.all_tasks()` will be removed in 3.9.

* Fix handshake timeout leak in asyncio/sslproto

Refs MagicStack/uvloop#222

* Break circular ref _SSLPipe <-> SSLProtocol

* bpo-34745: Fix asyncio ssl memory leak

* Break circular ref SSLProtocol <-> UserProtocol

* Add NEWS entry

asyncio.ProactorEventLoop now catchs and logs send errors when the
self-pipe is full: BaseProactorEventLoop._write_to_self() now catchs
and logs OSError exceptions, as done by
BaseSelectorEventLoop._write_to_self().

Add repr(self) to the log to display the number of pending overlapped
in the log.

IocpProactor.close() now uses time to decide when to log: wait 1
second before the first log, then log every second. Log also the
number of seconds since close() was called.

* asyncio: __del__() keep reference to warnings.warn

The __del__() methods of asyncio classes now keep a strong reference
to the warnings.warn() to be able to display the ResourceWarning
warning in more cases. Ensure that the function remains available if
instances are destroyed late during Python shutdown (while module
symbols are cleared).

* Rename warn parameter to _warn

"_warn" name is a hint that it's not the regular warnings.warn()
function.

* _wait_for_handle(), _register() and _unregister() methods of
  IocpProactor now raise an exception if closed
* Add "closed" to IocpProactor.__repr__()
* Simplify IocpProactor.close()

bpo-32622, bpo-35682: Fix asyncio.ProactorEventLoop.sendfile(): don't
attempt to set the result of an internal future if it's already done.

Fix asyncio _ProactorBasePipeTransport._force_close(): don't set the
result of _empty_waiter if it's already done.