Duplicate file is closed when original file is closed

[slozier]

From IronLanguages/ironpython2#92:

import os

with open("temp.txt", "wb+") as f:
    f.write(b"test")
    f.seek(os.SEEK_SET, 0)
    x = os.dup(f.fileno())
    g = os.fdopen(x, "rb")

assert g.read() == b"test"

fails with:

io.UnsupportedOperation: File not open for reading