pty_helper.py doesn't work with python3

[sam-github]

In process of
#25789 this is seen:

make test PYTHON=python3

Path: pseudo-tty/test-tty-color-support
Traceback (most recent call last):
  File "/home/sam/w/core/lts/test/pseudo-tty/pty_helper.py", line 97, in <module>
    if pipe(parent_fd, STDOUT):
  File "/home/sam/w/core/lts/test/pseudo-tty/pty_helper.py", line 33, in pipe
    n = os.write(dfd, data)
TypeError: a bytes-like object is required, not 'filter'

Affects most(all?) pseudo-tty tests.

/cc @cclauss @bnoordhuis @nodejs/python

[cclauss]

Python 2 does not distinguish between bytes and strings but Py3 strictly enforces it.

See to_utf8() Etc.