Python issue #21645, Tulip issue 192: Rewrite signal handling

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.
(Merge 3.4) Python issue #21645, Tulip issue 192: Rewrite signal handling

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.
Issue #21247: Fix a race condition in test_send_signal() of asyncio

Add a basic synchronization mechanism to wait until the child process is ready
before sending it a signal.
(Merge 3.4) Issue #21247: Fix a race condition in test_send_signal() of asyncio

Add a basic synchronization mechanism to wait until the child process is ready
before sending it a signal.
Fix asyncio.__all__: export also unix_events and windows_events symbols

For example, on Windows, it was not possible to get ProactorEventLoop or
DefaultEventLoopPolicy using "from asyncio import *".
(Merge 3.4) Fix asyncio.__all__: export also unix_events and windows_events
symbols

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