userns-block-fd: Add support for Python 3

According to PEP 394, the python command is meant to be Python 2
until at least 2020, so in practice this script will be run with
Python 2 for now (except on Arch Linux); but it seems good to be
more future-proof.

In Python 3, os.write() takes a bytestring (bytes object), not a
text string (str/unicode object). In Python 2 ≥ 2.6, the b'' syntax
is supported and gives a str object, because that was a bytestring
in Python 2; either way, b'1' is an acceptable argument to os.write().

In Python ≥ 3.4, the result of os.pipe() is close-on-exec
(non-inheritable) by default, so undo that where needed.

Signed-off-by: Simon McVittie <smcv@collabora.com>

Closes: #246
Approved by: giuseppe

demos/userns-block-fd.py

@@ -1,6 +1,6 @@
 #!/usr/bin/env python
 
-import os, select, subprocess, json
+import os, select, subprocess, sys, json
 
 pipe_info = os.pipe()
 userns_block = os.pipe()
@@ -19,11 +19,15 @@ if pid != 0:
     subprocess.call(["newuidmap", child_pid, "0", str(os.getuid()), "1"])
     subprocess.call(["newgidmap", child_pid, "0", str(os.getgid()), "1"])
 
-    os.write(userns_block[1], '1')
+    os.write(userns_block[1], b'1')
 else:
     os.close(pipe_info[0])
     os.close(userns_block[1])
 
+    if sys.version_info >= (3, 4):
+        os.set_inheritable(pipe_info[1], True)
+        os.set_inheritable(userns_block[0], True)
+
     args = ["bwrap",
             "bwrap",
             "--unshare-all",