I) Manual test for subprocess32:

janusz@nc6120:~$ python
Python 2.7.12 (default, Nov 19 2016, 06:48:10) 
[GCC 5.4.0 20160609] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import subprocess32
>>> cmd = ["ssh", "root@localhost", "-p", "50494", "whoami"]
>>> buf = subprocess32.check_output(cmd, stderr=subprocess32.STDOUT)
>>> print buf
root

>>>
janusz@nc6120:~$ ssh root@localhost -p 50494 -O exit
Exit request sent.
janusz@nc6120:~$ python
Python 2.7.12 (default, Nov 19 2016, 06:48:10) 
[GCC 5.4.0 20160609] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import subprocess32
>>> cmd = ["ssh", "root@localhost", "-p", "50494", "whoami"]
>>> buf = subprocess32.check_output(cmd, stderr=subprocess32.STDOUT)
after 20 seconds
^CTraceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/dist-packages/subprocess32.py", line 628, in check_output
    output, unused_err = process.communicate(timeout=timeout)
  File "/usr/lib/python2.7/dist-packages/subprocess32.py", line 915, in communicate
    stdout = _eintr_retry_call(self.stdout.read)
  File "/usr/lib/python2.7/dist-packages/subprocess32.py", line 543, in _eintr_retry_call
    return func(*args)
KeyboardInterrupt
>>>

II) When omit stderr:

janusz@nc6120:~$ ssh root@localhost -p 50494 -O exit
Exit request sent.
janusz@nc6120:~$ python
Python 2.7.12 (default, Nov 19 2016, 06:48:10) 
[GCC 5.4.0 20160609] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import subprocess32
>>> cmd = ["ssh", "root@localhost", "-p", "50494", "whoami"]
>>> buf = subprocess32.check_output(cmd)
>>> print buf
root

>>>

Strace for stderr=None:

mmap2(NULL, 262144, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7071000
close(6)                                = 0
close(5)                                = 0
close(4)                                = 0
ugetrlimit(RLIMIT_NOFILE, {rlim_cur=1024, rlim_max=64*1024}) = 0
close(3)                                = 0
pipe2([3, 4], O_CLOEXEC)                = 0
fstat64(3, {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
fcntl64(3, F_GETFL)                     = 0 (flags O_RDONLY)
pipe2([5, 6], O_CLOEXEC)                = 0
clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0xb74e3768) = 2728
close(6)                                = 0
close(4)                                = 0
brk(0x93d2000)                          = 0x93d2000
read(5, "", 50000)                      = 0
brk(0x93c6000)                          = 0x93c6000
close(5)                                = 0
fstat64(3, {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
_llseek(3, 0, 0xbfe23f90, SEEK_CUR)     = -1 ESPIPE (Illegal seek)
read(3, "root\n", 6)                    = 5
read(3, "", 1)                          = 0
--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=2728, si_uid=1000, si_status=0, si_utime=6, si_stime=0} ---
close(3)                                = 0
waitpid(2728, [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], 0) = 2728
rt_sigaction(SIGINT, {SIG_DFL, [], 0}, {0x8183c10, [], 0}, 8) = 0
brk(0x93c4000)                          = 0x93c4000
exit_group(0)                           = ?
+++ exited with 0 +++