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When building python extensions in the background w/o stdin (and stderr and stdout redirected to a log file), the invocation of setup.py fails.
Normal build in a shell:
Example from pyexpat:
> /usr/pkg/bin/python3.6 setup.py build
running build
running build_ext
building 'pyexpat' extension
creating build
creating build/temp.netbsd-8.99.14-amd64-3.6
creating build/temp.netbsd-8.99.14-amd64-3.6/Modules
gcc -DNDEBUG -O2 -pipe -D_FORTIFY_SOURCE=2 -I/usr/include -I/usr/pkg/include -O2 -pipe -D_FORTIFY_SOURCE=2 -I/usr/include -I/usr/pkg/include/python3.6 -I/usr/include -I/usr/pkg/include/python3.6 -fPIC -DHAVE_EXPAT_H -I/work/pkgobj/textproc/py-expat/work/.buildlink/include -I/usr/pkg/include/python3.6 -c Modules/pyexpat.c -o build/temp.netbsd-8.99.14-amd64-3.6/Modules/pyexpat.o
creating build/lib.netbsd-8.99.14-amd64-3.6
gcc -pthread -shared -L. -L/usr/lib -Wl,-R/usr/lib -L/usr/pkg/lib -Wl,-R/usr/pkg/lib -L/usr/X11R7/lib -Wl,-R/usr/X11R7/lib -L/usr/lib -Wl,-R/usr/lib -L/usr/pkg/lib -Wl,-R/usr/pkg/lib -L/usr/X11R7/lib -Wl,-R/usr/X11R7/lib -L/usr/lib -Wl,-R/usr/lib -Wl,-R/usr/pkg/lib -O2 -pipe -D_FORTIFY_SOURCE=2 -I/usr/include -I/usr/pkg/include/python3.6 -I/usr/include -I/usr/pkg/include/python3.6 build/temp.netbsd-8.99.14-amd64-3.6/Modules/pyexpat.o -L/work/pkgobj/textproc/py-expat/work/.buildlink/lib -L/usr/pkg/lib -Wl,-R/work/pkgobj/textproc/py-expat/work/.buildlink/lib -lexpat -lpython3.6 -o build/lib.netbsd-8.99.14-amd64-3.6/pyexpat.so
But if invoked as 
> ( sleep 15; /usr/pkg/bin/python3.6 setup.py build ) >/tmp/log 2>&1 &
> ^D
and then watching /tmp/log from another session:
Fatal Python error: Py_Initialize: can't initialize sys standard streams
OSError: [Errno 9] Bad file descriptor
Current thread 0x000075b4e4cee800 (most recent call first):
[1] Abort trap (core dumped) /usr/pkg/bin/pyt...

I cannot reproduce the problem with Python 3.6.5, building pyephem and with the command:
( sleep 15; python setup.py build ) >foo.log 2>&1

You need to exit the parent shell, to get the original stdin revoke(2)'d.
That is: the Ctrl-D in the original descritpion is not line noise. Sorry, should have been more explicit (or used "exit" or something instead)

A simpler way to reproduce that (probably this problem is not limited to
building but to sys module):
 (sleep 10; python3.6 -c 'import sys' >/tmp/log 2>&1) & exit
And in `/tmp/log':
 Fatal Python error: Py_Initialize: can't initialize sys standard streams
 OSError: [Errno 9] Bad file descriptor
 
 Current thread 0x00007226e0ab3800 (most recent call first):

And here the backtrace of the corresponding core (this is on
NetBSD/amd64 8.99.15 with lang/python36 package of today pkgsrc-current):
 % gdb -core python3.6.core `which python3.6`
 Reading symbols from /usr/pkg/bin/python3.6...done.
 [New process 1]
 Core was generated by `python3.6'.
 Program terminated with signal SIGABRT, Aborted.
 #0 0x000076255932924a in _lwp_kill () from /usr/lib/libc.so.12
 (gdb) bt
 #0 0x000076255932924a in _lwp_kill () from /usr/lib/libc.so.12
 #1 0x0000762559328e65 in abort () at /usr/src/lib/libc/stdlib/abort.c:74
 #2 0x000076255a749d33 in Py_FatalError (msg=msg@entry=0x76255a7dea20 "Py_Initialize: can't initialize sys standard streams")
 at Python/pylifecycle.c:1462
 #3 0x000076255a74af4c in _Py_InitializeEx_Private (install_sigs=1, install_importlib=1) at Python/pylifecycle.c:450
 #4 0x000076255a764902 in Py_Main (argc=argc@entry=3, argv=argv@entry=0x76255ab01060) at Modules/main.c:700
 #5 0x0000000000400e2f in main (argc=3, argv=<optimized out>) at ./Programs/python.c:69
 (gdb) f 4
 #4 0x000076255a764902 in Py_Main (argc=argc@entry=3, argv=argv@entry=0x76255ab01060) at Modules/main.c:700
 700 Py_Initialize();
 (gdb) f 3
 #3 0x000076255a74af4c in _Py_InitializeEx_Private (install_sigs=1, install_importlib=1) at Python/pylifecycle.c:450
 450 Py_FatalError(
 (gdb) list
 445 if (_PyTraceMalloc_Init() < 0)
 446 Py_FatalError("Py_Initialize: can't initialize tracemalloc");
 447
 448 initmain(interp); /* Module __main__ */
 449 if (initstdio() < 0)
 450 Py_FatalError(
 451 "Py_Initialize: can't initialize sys standard streams");
 452
 453 /* Initialize warnings. */
 454 if (PySys_HasWarnOptions()) {
Please let me know if any further information is needed and I will try
to help!
Thank you!

After testing is_valid_fd() (from Python/pylifecycle.c) separately
(an `is_valid_fd.c' file will be attached to ease reproduction) I think
that also NetBSD is affected by bpo-30225.
Using dup(2) (at is currently done in NetBSD):
 % cc -o ivf is_valid_fd.c
 % sleep 5 && ./ivf > /tmp/log & exit
 [... the terminal is closed via ^D ...]
 % cat /tmp/log
 0: 1
 1: 1
 2: 1
Using fstat(2):
 % cc -DUSE_FSTAT -o ivf is_valid_fd.c
 % sleep 5 && ./ivf > /tmp/log & exit
 [... the terminal is closed via ^D ...]
 % cat /tmp/log
 0: 0
 1: 1
 2: 0
The possible attached patch seems to fix the problem reported by Martin.

I'm quite that this issue has been fixed by bpo-30225 which is part Python 3.7.4, Python 3.8.0 and newer. I mark the issue as duplicate of bpo-30225. Please reopen the bug if these versions still fail on NetBSD.
commit 1c4670ea0cc3d208121af11b9b973e6bb268e570
Author: Victor Stinner <victor.stinner@gmail.com>
Date: Thu May 4 00:45:56 2017 +0200
 bpo-30225: Fix is_valid_fd() on macOS Tiger (#1443)
 
 is_valid_fd() now uses fstat() instead of dup() on macOS to return 0
 on a pipe when the other side of the pipe is closed. fstat() fails
 with EBADF in that case, whereas dup() succeed.

Hello Victor,
I can confirm that I can no longer reproduce this problem with Python
3.7.5 on NetBSD and according a double-check to
Python/pylifecycle.c:is_valid_fd() now fstat() is always used on
NetBSD.
Thanks for the update and for looking at it!

> I can confirm that I can no longer reproduce this problem with Python
3.7.5 on NetBSD
Great!