Steps to reproduce:

1. Use a Windows, with system default encoding to cp936 (Chinese PRC, or
Simplified Chinese) in Regional Options.
2. Open Python 3.0 (command line).
3. Type:
import os
import sys
os.system(("echo " + sys.stdin.readline().rstrip("\n")).encode("cp936"))
(in stdin type:)
我

Result:
The output from "echo" would be utf-8 mistakenly used as cp936:
鎴?

Expected result:
The "echo" command outputs "我".

Comments:
I guess os.system can recoding the string before sending the string out.
This may be done in the C part of Python. BTW, The os.popen() function 
is
correct.