In Python 2.7 setting PYTHONIOENCODING=undefined mostly works as expected.

$ PYTHONIOENCODING=undefined python -c 'print(123)'
123
$ PYTHONIOENCODING=undefined python -c 'print("abc")'
abc
$ PYTHONIOENCODING=undefined python -c 'print(u"abc")'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib/python2.7/encodings/undefined.py", line 19, in encode
    raise UnicodeError("undefined encoding")
UnicodeError: undefined encoding

Most tests (except test_doctest and test_gdb) are passed with PYTHONIOENCODING=undefined.

But in Python 3 setting PYTHONIOENCODING=undefined seems just silently terminates the interpreter (exited with code 1).