Issue #20400: Merge Tulip into Python: add the new asyncio.subprocess module

* Add a new asyncio.subprocess module
* Add new create_subprocess_exec() and create_subprocess_shell() functions
* The new asyncio.subprocess.SubprocessStreamProtocol creates stream readers
  for stdout and stderr and a stream writer for stdin.
* The new asyncio.subprocess.Process class offers an API close to the
  subprocess.Popen class:

  - pid, returncode, stdin, stdout and stderr attributes
  - communicate(), wait(), send_signal(), terminate() and kill() methods

* Remove STDIN (0), STDOUT (1) and STDERR (2) constants from base_subprocess
  and unix_events, to not be confused with the symbols with the same name of
  subprocess and asyncio.subprocess modules
* _ProactorBasePipeTransport.get_write_buffer_size() now counts also the size
  of the pending write
* _ProactorBaseWritePipeTransport._loop_writing() may now pause the protocol if
  the write buffer size is greater than the high water mark (64 KB by default)

Lib/asyncio/unix_events.py

@@ -21,16 +21,11 @@ from . import transports
 from .log import logger
 
 
-__all__ = ['SelectorEventLoop', 'STDIN', 'STDOUT', 'STDERR',
+__all__ = ['SelectorEventLoop',
            'AbstractChildWatcher', 'SafeChildWatcher',
            'FastChildWatcher', 'DefaultEventLoopPolicy',
            ]
 
-STDIN = 0
-STDOUT = 1
-STDERR = 2
-
-
 if sys.platform == 'win32':  # pragma: no cover
     raise ImportError('Signals are not really supported on Windows')