Merge (manually) the subprocess_stream into default

* 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)
* Add new subprocess examples: shell.py, subprocess_shell.py,
* subprocess_attach_read_pipe.py and subprocess_attach_write_pipe.py

1 files changed, 3 insertions, 1 deletions

diff --git a/examples/child_process.py b/examples/child_process.py
index 4410414..0c12cb9 100644
--- a/examples/child_process.py
+++ b/examples/child_process.py
@@ -1,7 +1,9 @@
 """
 Example of asynchronous interaction with a child python process.
 
-Note that on Windows we must use the IOCP event loop.
+This example shows how to attach an existing Popen object and use the low level
+transport-protocol API. See shell.py and subprocess_shell.py for higher level
+examples.
 """
 
 import os