Oh, I reproduced the bug.
------------------------
[3021] test_threadsignals
test_signals (test.test_threadsignals.ThreadSignals) ... test_signals: acquire lock (thread -1610559488)
test_signals: wait lock (thread -1610559488)
send_signals: enter (thread 27159552)
send_signals: raise SIGUSR1
send_signals: raise SIGUSR2
send_signals: release signalled_all
send_signals: exit (thread 27159552)

^C
------------------------

There are 2 threads: faulthandler thread and the mainthread. Trace of the main thread:
-------
(gdb) where
#0  0xffff027a in ___spin_lock () at /System/Library/Frameworks/System.framework/PrivateHeaders/i386/cpu_capabilities.h:216
#1  0x90001433 in pthread_mutex_lock ()
#2  0x0011b34c in PyThread_acquire_lock_timed (lock=0x1124f10, microseconds=0, intr_flag=0) at Python/thread_pthread.h:450
#3  0x0011b73e in PyThread_acquire_lock (lock=0x1124f10, waitflag=0) at Python/thread_pthread.h:531
#4  0x000775b2 in Py_AddPendingCall (func=0x713dd <checksignals_witharg>, arg=0x0) at Python/ceval.c:509
#5  0x00071445 in signal_handler (sig_num=31) at ./Modules/signalmodule.c:189
#6  <signal handler called>
#7  0x9010d494 in spin_unlock ()
#8  0x90001620 in pthread_mutex_lock ()
#9  0x0011b34c in PyThread_acquire_lock_timed (lock=0x1124f10, microseconds=0, intr_flag=0) at Python/thread_pthread.h:450
#10 0x0011b73e in PyThread_acquire_lock (lock=0x1124f10, waitflag=0) at Python/thread_pthread.h:531
#11 0x000775b2 in Py_AddPendingCall (func=0x713dd <checksignals_witharg>, arg=0x0) at Python/ceval.c:509
#12 0x00071445 in signal_handler (sig_num=30) at ./Modules/signalmodule.c:189
#13 <signal handler called>
#14 0x900248c7 in semaphore_wait_signal_trap ()
#15 0x900288b4 in pthread_cond_wait ()
#16 0x0011b52c in PyThread_acquire_lock_timed (lock=0x1171180, microseconds=-1, intr_flag=1) at Python/thread_pthread.h:476
#17 0x00180746 in acquire_timed (lock=0x1171180, microseconds=-1) at ./Modules/_threadmodule.c:66
#18 0x001809aa in lock_PyThread_acquire_lock (self=0x14ff820, args=0x512038, kwds=0x0) at ./Modules/_threadmodule.c:133
#19 0x0005f58e in PyCFunction_Call (func=0x166c5f8, arg=0x512038, kw=0x0) at Objects/methodobject.c:84
#20 0x0008c1cd in call_function (pp_stack=0xbfff5f64, oparg=0) at Python/ceval.c:3859
#21 0x000869dd in PyEval_EvalFrameEx (f=0x1193798, throwflag=0) at Python/ceval.c:2657
...
-------

The main thread was waiting test_signals() lock (signalled_all) while it is was interrupted by a signal. The signal handler calls Py_AddPendingCall() which blocks on acquiring "pending_lock".