Syscalls made by open("/dev/stdout", "a") in Python 2:

  open("/dev/stdout", O_WRONLY|O_CREAT|O_APPEND, 0666) = 3
  lseek(3, 0, SEEK_END)                   = -1 ESPIPE (Illegal seek)
  fstat(3, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 0), ...}) = 0

I used this comand:

  $ strace -o trace python2 -c 'import os; os.uname();os.uname();os.uname(); f=open("/dev/stdout", "a"); os.uname(); f.close()'

It looks like the C library simply ignores ESPIPE on lseek(fd, 0, SEEK_END) when opening a file in append mode:
https://sourceware.org/git/?p=glibc.git;a=blob;f=libio/fileops.c;h=13157354272ff9ab1832d4a619a81f05898fcd69;hb=HEAD#l242

  if ((read_write & _IO_IS_APPENDING) && (read_write & _IO_NO_READS))
    if (_IO_SEEKOFF (fp, (_IO_off64_t)0, _IO_seek_end, _IOS_INPUT|_IOS_OUTPUT)
	== _IO_pos_BAD && errno != ESPIPE)
      {
	close_not_cancel (fdesc);
	return NULL;
      }

from _IO_file_open() file operation.