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/test/test_asyncio/test_windows_events.py

@@ -105,7 +105,7 @@ class ProactorTests(unittest.TestCase):
         self.loop.run_until_complete(f)
         elapsed = self.loop.time() - start
         self.assertFalse(f.result())
-        self.assertTrue(0.18 < elapsed < 0.9, elapsed)
+        self.assertTrue(0.18 < elapsed < 0.5, elapsed)
 
         _overlapped.SetEvent(event)