Actually, I think I *am* seeing this on 3.7:

(gdb) bt
#0  0x00007f130d530adb in futex_abstimed_wait (cancel=true, private=<optimized out>, abstime=0x0, expected=0,
    futex=0x142c6d0) at ../nptl/sysdeps/unix/sysv/linux/sem_waitcommon.c:43
#1  do_futex_wait (sem=sem@entry=0x142c6d0, abstime=0x0) at ../nptl/sysdeps/unix/sysv/linux/sem_waitcommon.c:223
#2  0x00007f130d530b6f in __new_sem_wait_slow (sem=0x142c6d0, abstime=0x0)
    at ../nptl/sysdeps/unix/sysv/linux/sem_waitcommon.c:292
#3  0x00007f130d530c0b in __new_sem_wait (sem=<optimized out>) at ../nptl/sysdeps/unix/sysv/linux/sem_wait.c:28
#4  0x0000000000535baa in PyThread_acquire_lock_timed (lock=0x142c6d0, microseconds=microseconds@entry=-1,
    intr_flag=intr_flag@entry=0) at Python/thread_pthread.h:340
#5  0x0000000000535c24 in PyThread_acquire_lock (lock=<optimized out>, waitflag=waitflag@entry=1)
    at Python/thread_pthread.h:576
#6  0x000000000058949a in _enter_buffered_busy (self=0x7f130ca0fbf8) at ./Modules/_io/bufferedio.c:282
#7  buffered_flush (self=0x7f130ca0fbf8, args=<optimized out>) at ./Modules/_io/bufferedio.c:835
#8  0x000000000043aae8 in _PyMethodDef_RawFastCallDict (kwargs=<optimized out>, nargs=0, args=<optimized out>,
    self=0x7f130ca0fbf8, method=0x8eeda0 <bufferedwriter_methods+320>) at Objects/call.c:482
#9  _PyCFunction_FastCallDict (func=0x7f130aeb3438, args=<optimized out>, nargs=0, kwargs=<optimized out>)
    at Objects/call.c:582