Fix detecting piped input

[Burrito-Bazooka]

Fixes case when used (through subprocess) inside Python script which is not run from a tty, I bet there are a few other cases it fixes as well.

edit: I actually encountered the behaviour while making a HexChat script which uses shell commands (among them, xerox). xerox behaved differently depending on whether I ran HexChat from a terminal, or from a desktop launcher. ^/e

edit2 (17 Jul): To be clear, the rest of this message is just to demonstrate why the current behaviour is buggy. They aren't files I want committed.^/e2

Files

isatty_test1.py (executable):

#!/usr/bin/env python3

import sys, stat, os

mode = os.fstat(0).st_mode
print(mode)

print("isatty", sys.stdin.isatty())

print("isfifo", stat.S_ISFIFO(mode))
print("isreg", stat.S_ISREG(mode))

isatty_test2.py

import subprocess
p = subprocess.Popen(['./isatty_test1.py'], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
print(p.communicate())

isatty_test3.py (executable - same as test2 but writes to a file so that we can run it without a tty):

#!/usr/bin/env python3

import subprocess
p = subprocess.Popen(['./isatty_test1.py'], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
with open("./test3.out", "w") as f:
    print(p.communicate(), file=f)

Tests

(on Linux, Debian testing (stretch/sid))

In a terminal ./isatty_test1.py
Output:

8592
isatty True
isfifo False
isreg False

In a terminal python3 ./isatty_test2.py
Output:

(b'8592\nisatty True\nisfifo False\nisreg False\n', b'')

Running isatty_test3.py as an executable from my file manager to capture buggy behaviour
(note that no input was piped in, but the first output doesn't match xerox's current expectations)
Output (test3.out):

(b'8630\nisatty False\nisfifo False\nisreg False\n', b'')

In terminal ./isatty_test1.py < test3.out
Output:

33188
isatty False
isfifo False
isreg True

In a terminal cat ./test3.out | ./isatty_test1.py
Output:

4480
isatty False
isfifo True
isreg False

[Burrito-Bazooka]

Hey, are there any problems with this PR? Not that I need it quickly.