Adds `load_package_tests` function to test.support, uses it in test_asyncio,
test_email, test_json, test_tools, test_importlib and all test_importlib
sub-packages to implement test discovery.

* Tulip issue 194: Don't use sys.getrefcount() in unit tests
* signal.set_wakeup_fd() can now raise an OSError on Python 3.5

specified.

Replace also _make_drain_waiter() function with a classic _drain_helper()
coroutine.

now display special message when and only when there are failures.

Thanks to William Orr.

Sort also imports in support/__init__.py

shutil.rmtree() to remove the temporary directory.

directory, not os.path.isdir()

For example, on Windows, it was not possible to get ProactorEventLoop or
DefaultEventLoopPolicy using "from asyncio import *".

Add a basic synchronization mechanism to wait until the child process is ready
before sending it a signal.

Since Python 3.3, the C signal handler writes the signal number into the wakeup
file descriptor and then schedules the Python call using Py_AddPendingCall().

asyncio uses the wakeup file descriptor to wake up the event loop, and relies
on Py_AddPendingCall() to schedule the final callback with call_soon().

If the C signal handler is called in a thread different than the thread of the
event loop, the loop is awaken but Py_AddPendingCall() was not called yet. In
this case, the event loop has nothing to do and go to sleep again.
Py_AddPendingCall() is called while the event loop is sleeping again and so the
final callback is not scheduled immediatly.

This patch changes how asyncio handles signals. Instead of relying on
Py_AddPendingCall() and the wakeup file descriptor, asyncio now only relies on
the wakeup file descriptor. asyncio reads signal numbers from the wakeup file
descriptor to call its signal handler.

ConnectionResetError

ConnectionResetError too

If you want to handle the BrokenPipeError, you can easily reimplement
communicate().

Add also a unit test to ensure that stdin.write() + stdin.drain() raises
BrokenPipeError.

attribute.  Based on patch by Martin Panter.

pydoc.gui was removed over 3 years ago (#10818), this script has been
utterly bereft of worth since then.

in gather().

The _SelectorSslTransport constructor already calls it.

Fix regression that this test would have caught.

in debug mode

* Tulip issue #184: Log subprocess events in debug mode

  - Log stdin, stdout and stderr transports and protocols
  - Log process identifier (pid)
  - Log connection of pipes
  - Log process exit
  - Log Process.communicate() tasks: feed stdin, read stdout and stderr
  - Add __repr__() method to many classes related to subprocesses


* Add BaseSubprocessTransport._pid attribute. Store the pid so it is still
  accessible after the process exited. It's more convinient for debug.

* create_connection(): add the socket in the "connected to" debug log

* Clean up some docstrings and comments. Remove unused unimplemented
  _read_from_self().