Interesting when using faster machine with newer ssh don't see this problem :)

janusz@t560:~$ ssh -V
OpenSSH_7.3p1 Ubuntu-1ubuntu0.1, OpenSSL 1.0.2g  1 Mar 2016

janusz@t560:~$ ssh root@localhost -O exit; strace python test.py

getrlimit(RLIMIT_NOFILE, {rlim_cur=1024, rlim_max=4*1024}) = 0
close(3)                                = 0
pipe2([3, 4], O_CLOEXEC)                = 0
fstat(3, {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
fcntl(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=0x7fb7ba6fc9d0) = 22284
close(6)                                = 0
close(4)                                = 0
brk(0xca73792000)                       = 0xca73792000
read(5, "", 50000)                      = 0
brk(0xca73786000)                       = 0xca73786000
close(5)                                = 0
fstat(3, {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
lseek(3, 0, 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=22284, si_uid=1000, si_status=0, si_utime=1, si_stime=0} ---
close(3)                                = 0
wait4(22284, [{WIFEXITED(s) && WEXITSTATUS(s) == 0}], 0, NULL) = 22284
rt_sigaction(SIGINT, {SIG_DFL, [], SA_RESTORER, 0x7fb7ba2f1630}, {0xca720ef1a0, [], SA_RESTORER, 0x7fb7ba2f1630}, 8) = 0
exit_group(0)                           = ?
+++ exited with 0 +++

So, two options here:
- new version of ssh
- faster CPU

Will try to update ssh on the old/slow machine.