The problem is:
If all file descriptors are closed, including the standard streams, the default /dev/null replacements which are opened in redirect_stream are not inheritable.

With python >= 3.4 all newly opened files are not inheritable by default.
https://docs.python.org/3/library/os.html#fd-inheritance

While dup2 would set the new file descriptor to be inheritable, it does that only if the file descriptors differ.
In the default case they don't differ because the standard streams have just been closed and the new /dev/null replacements get the 0-2 descriptors since they are the lowest.
If the file descriptors that should be redirected don't differ, the inheritable flag needs to be set manually.
If the inheritable flag is not set on the standard streams, child process are started without any standard streams.

The following snippet shows the missing stdout file descriptor in a child process (OSError: [Errno 9] Bad file descriptor: 1)

#!/usr/bin/python3
from daemon.daemon import DaemonContext
import subprocess
import sys
daemon = DaemonContext(stderr=sys.stderr)
daemon.open()
subprocess.call(['python3', '-c', 'import sys; import os; print(os.stat(1), file=sys.stderr)']

Thanks :)