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)

1 files changed, 2 insertions, 0 deletions

diff --git a/Lib/asyncio/__init__.py b/Lib/asyncio/__init__.py
index eb22c38..3df2f80 100644
--- a/Lib/asyncio/__init__.py
+++ b/Lib/asyncio/__init__.py
@@ -24,6 +24,7 @@ from .locks import *
 from .protocols import *
 from .queues import *
 from .streams import *
+from .subprocess import *
 from .tasks import *
 from .transports import *
 
@@ -39,5 +40,6 @@ __all__ = (events.__all__ +
            protocols.__all__ +
            queues.__all__ +
            streams.__all__ +
+           subprocess.__all__ +
            tasks.__all__ +
            transports.__all__)