Issue #21817: When an exception is raised in a task submitted to a ProcessPoolExecutor, the remote traceback is now displayed in the parent process.
Patch by Claudiu Popa.

Patch by Nikolaus Rath and Martin Panter.

Only happened with oddly capitalized debug executables on Windows.
Patch by Claudiu Popa.

Patch by Jon Dufresne.

Override the connect_read_pipe() method of the loop to mock immediatly
pause_reading() and resume_reading() methods.

The test failed randomly on FreeBSD 9 buildbot and on Windows using trollius.

StreamWriter: close() now clears the reference to the transport

StreamWriter now raises an exception if it is closed: write(), writelines(),
write_eof(), can_write_eof(), get_extra_info(), drain().

Closes #23160: Respect the environment variable SVNROOT in external-common.bat (patch by anselm.kruis)

* Use test_utils.run_briefly() to execute pending calls to really close
  transports
* sslproto: mock also _SSLPipe.shutdown(), it's need to close the transport
* pipe test: the test doesn't close explicitly the PipeHandle, so ignore
  the warning instead
* test_popen: use the context manager ("with p:") to explicitly close pipes

Set the _read_fut attribute to None after cancelling it.

This change should fix a race condition with
_ProactorWritePipeTransport._pipe_closed().

Check if the _sock attribute is None to check if the transport is closed.

Do nothing if the transport is already closed. Before it was not possible to
close the transport twice.

transport at subprocess exit.

Clear also its reference to the transport.

Don't call immediatly self._process_write_backlog() but schedule the call using
call_soon(). _on_handshake_complete() can be called indirectly from
_process_write_backlog(), and _process_write_backlog() is not reentrant.

StreamWriter now raises an exception if it is closed: write(), writelines(),
write_eof(), can_write_eof(), get_extra_info(), drain().