* bpo-31034: Reliable signal handler for test_asyncio

Don't rely on the current SIGHUP signal handler, make sure that it's
set to the "default" signal handler: SIG_DFL.

* Add comments

* bpo-30280: asyncio now cleans up threads

asyncio base TestCase now uses threading_setup() and
threading_cleanup() of test.support to cleanup threads.

* asyncio: Fix TestBaseSelectorEventLoop cleanup

bpo-30280: TestBaseSelectorEventLoop of
test.test_asyncio.test_selector_events now correctly closes the event
loop: cleanup its executor to not leak threads.

Don't override the close() method of the event loop, only override
the_close_self_pipe() method.

Revert "bpo-29406: asyncio SSL contexts leak sockets after calling close with certain servers (#409)" (#2111)

This reverts commit a608d2d5.

* asyncio SSL contexts leak sockets after calling close with certain servers

* cleanup _shutdown_timeout_handle on _fatal_error

Avoid a deadlock when the waiter who is about to take the lock is
cancelled

Issue #27585

when there are no more `await` or `yield (from)` before return in coroutine,
cancel was ignored.

example:

    async def coro():
        asyncio.Task.current_task().cancel()
        return 42
    ...
    res = await coro()  # should raise CancelledError

Make also minor PEP8 coding style fixes on modified imports.

Allow AF_UNIX in create_server & create_connection

Some built-in coroutine-like objects might not have __name__ or
__qualname__. A good example of such are 'asend', 'aclose' and
'athrow' coroutine methods of asynchronous generators.

when called from coroutines or callbacks.

Run expensive type checks only in debug mode.  In addition, stop
supporting passing handles to loop.run_in_executor.

This implementation provides additional 10-20% speed boost for
asyncio programs.

The patch also fixes _asynciomodule.c to use Arguments Clinic, and
makes '_schedule_callbacks' an overridable method (as it was in 3.5).

Also, add 'isfuture' to 'asyncio.futures.__all__', so that it's
exposed as 'asyncio.isfuture'.

Patch by Johannes Ebke.

Patch by Коренберг Марк.