* 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)
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